Wikipedia:Reference desk/Archives/Computing/2015 October 18

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October 18

Unitary transformation

Is there any unitary transformation that satisfies: $\exists |z\rangle \in \{|0\rangle ,|1\rangle \}^{n}\ \forall |a\rangle ,|b\rangle \in \{|0\rangle ,|1\rangle \}\ \exists |z'\rangle \in \{|0\rangle ,|1\rangle \}^{n}\ T(|a0z\rangle +|b1z\rangle )=(|a\rangle +|b\rangle )|z'\rangle$ ? 185.32.179.137 (talk) 05:25, 18 October 2015 (UTC)

Seems that it's only Hadamard on the second qubit... 185.32.179.137 (talk) 05:41, 18 October 2015 (UTC)

Laptop hard drive passes CHKDSK and all SeaTools tests except long generic

If a laptop hard drive (Hitachi, in this case) passes CHKDSK without a hitch, as well as all tests in the SeaTools program (S.M.A.R.T check, short drive self check, short generic) except for long generic, does this still mean that the hard drive is close to failure? Also, if a hard drive occasionally makes a low-volume but still noticeable "grinding" noise, is it close to failure? For reference, the laptop in question once hanged and showed a Windows 8 BSOD saying that a "Kernel Data Inpage Error" occurred, then after restarting showed an "operating system not found" error, although it worked fine again after another restart. Narutolovehinata5 ^tccsdnew 06:43, 18 October 2015 (UTC)

Are all these the same hard drive? If so, yes, it's living on borrowed time. Random system freezes and the BIOS not finding a bootable disk fall firmly into the realm of Things That Shouldn't Ever Happen. Add random drive noises and drive test failures, and your drive is basically telling you with alarms and flashing lights that it's about to die. --71.119.131.184 (talk) 09:40, 18 October 2015 (UTC)

The thing is, the system freeze/"operating system not found error" only ever happened once. What I don't get is that as I mentioned previously it failed a Long Generic test but passed all the other tests and the CHKDSK did not find any bad sectors. Also, I've noticed that the hard disk sometimes makes read/write noises, particularly if I plug the laptop to a power adapter or remove it. Narutolovehinata5 ^tccsdnew 10:08, 18 October 2015 (UTC)

It generally worth looking at the log of whatever tool you're using and seeing precisely why the long generic test failed. (If you're tool doesn't have one, look for a better tool. IIRC, both WD & Seagate tools definitely have logs.) It's also often worth using Speedfan's online SMART comparison to try and get an idea of how your HD is faring, or at least some tool which can give more info about it if you don't know what you're looking at (for example, does the HD have any reallocate sectors). Speedfan also makes it easy to keep a record if you keep copy of the URL somewhere for future comparison. However with everything else mentioned, I agree with the IP the HD sounds to be on borrowed time. Nil Einne (talk) 10:27, 18 October 2015 (UTC)

@Nil Einne: Unfortunately, SeaTools for Windows (which is what I'm using) doesn't really give a full log: while it does give a log, it only shows which tests have passed or failed, not the actual test results. What is really strange is that the only test it failed is the Long Generic test; all other tests (SMART, SDT, Short Generic) passed, while a CHKDSK showed that there were no bad sectors. Looking online, it seems that normally, if a drive is close to failure, it would fail most if not all the tests rather than just one (i.e. a failing HDD would fail not just the long generic test but also generally the SMART and SDT tests as well). What could be the problem given it failed just the Long Generic test and not any of the other tests? Narutolovehinata5 ^tccsdnew 12:39, 18 October 2015 (UTC)

I don't think the results are as surprising as you test. Short generic and SDT only test a small proportion of the disk so could easily miss whatever it is causing the problem. You only get a SMART failure if one of the values has fallen below the manufacturers set limits, you can have disks which are clearly on their death door, without a SMART trip. (I think our article may say something similar.) CHKDSK will only report bad sectors if the HD has ran out of spare sectors to reassign failing ones to. As for the log, I thought even Seatools for Windows will provide more info, but I could be mistaken. Are you looking at the actual log file (normally stored in the Seatools program directory IIRC), rather than simply what Seatools is showing, in case it is hiding something? Nil Einne (talk) 02:14, 19 October 2015 (UTC)

An update: I downloaded a program called SpeedFan to check the actual SMART data. The "Uncorrectable Sector Count" attribute shows up as "000000000000" or 0, "Current Pending Sector Count" (which shows up as a warning) gives a value of "000000000008" or 8, and "Reallocated Event Count" gives "00000000000D" or 13. Is this bad? Narutolovehinata5 ^tccsdnew 19:04, 18 October 2015 (UTC)

One could argue that any value different from 0 is bad, otoh, the disk may run for years without developing more bad sectors. Comes down to how valuable your data is, whether you take backups, and whether you can afford a sudden failure (do you use the laptop for your job?).

According to this, the long test will fail "if a bad sector is detected on an internal drive". Passing the SMART test only means none of the values have reached the treshold value. As I understand it, "short generic" tests the inner, outer, and some random sectors of the disk, only the "long generic" tests all sectors. CHKDSK not finding any bad sectors may be because (depending on the mode or parameters used) it only tests the file system or because it doesn't actually test the physical sectors: bad sectors will have been reallocated by the disk itself, so Chkdsk isn't aware of them.

The grinding noises you sometimes hear could be windows programs like indexing running in the background. Whether the "operating system not found" was a hardware or software/BIOS problem, I'm not sure...

You have to weigh the trouble and cost of replacing the disk versus the cost of a disk failure and loss of data. If you prefer to be safe rather than sorry, you replace it. Ssscienccce (talk) 21:45, 18 October 2015 (UTC)

I see Ssscienccce has already said much of what I said above. One thing I will add is my assumption when you said "grinding" noises is you meant noises you have not heard before. Modern hard disks can sometimes make some fairly weird noises in normal operation which can be difficult for someone not experience with the particular hard disk model to know are normal, but if you suddenly get new noises you've never heard before, that's usually a bad sign. Also, while it's impossible to be certain that the "operating system not found" error wasn't due to some other hardware or software problem, when taken with the other things the most likely possibility is it was related to hard disk problems.

At the very least, you should run a fix all long [1], to ensure all pending sectors are reallocated. Any data in these sectors is basically unrecoverable anyway except perhaps if you're willing to spend thousands of dollars. There's a slight chance fix all long may kill the hard disk for good, then again so could chkdsk or a long generic. Once you've run fix all long, try rerunning a long generic again, I think it will probably pass then. If it doesn't you can try running fix all long again. However I would take that as another sign the hard disk is really dying. If fix all long fixes everything and a long generic passes, at a minimum you'd still want to keep an eye on the reallocated sectors, and probably rerun a long generic after a few weeks.

I presume you already have a backup of all important data (as you should even if the hard disk wasn't showing signs of having problems). So the main issues would be whether you want to keep running with the risk the hard disk may fail and you'd need to replace it at a less opportune time (i.e. you really need the computer and don't have time to replace the hard disk). And also the greater difficulties replacing the hard disk if it does fail (currently you could likely simply clone the disk on to another, and the system should work, if it fails, you can't). Plus the loss of any replaceable data you felt wasn't worth backing up (downloads for example).

Nil Einne (talk) 02:27, 19 October 2015 (UTC)

@Nil Einne: @Ssscienccce: There are a few things I need to clarify. First, I never listened to the hard disk before so I have no idea if the grinding noise is normal or not, although observing it now there are times when occasionally it stops. Second, I don't back up files, for the simple reason that I don't have an external hard disk: the drive I'm talking about is the laptop's built-in drive. Next, for some reason (perhaps because of what was previously mentioned), when I open SeaTools, the option to "Fix All" simply doesn't appear. Then, the hard drive itself (as well as the laptop, which is a Sony Vaio) is less than three years old (which as far as I know isn't exactly old for these kinds of things; my previous laptop, a Toshiba, was still working albeit very slowly when I replaced it, even though it was almost five years old). The laptop itself sometimes suddenly turns off (with a clicking noise from the laptop) when plugged in a certain way, although a technician told me it appears to be because of an unrelated electrical error. Finally, the aforementioned BSOD occurred while I was using Windows Media Player; at first, the laptop suddenly slowed down, which initially I attributed to Chrome, which lead me to close tabs, but the slow down did not stop and after a few minutes it showed the BSOD. Since then, WMP has worked fine, but that lead me to become worried about the hard drive. Narutolovehinata5 ^tccsdnew 03:37, 19 October 2015 (UTC)

If you don't have any backups, and you have anything on your computer you can't afford to lose, you really need to start right now. I'm not sure what magnitude of data you're talking about. If it's really small, then perhaps a free online service will be fine like Microsoft Onedrive or Google Drive. (You should normally have two backups in different locations, at least one which isn't online, but since you have none at the moment, starting with one will leave you in a far better position than you are now.) Alternatively a USB key or similar. If it's more then you may need an external or portable drive. (If you're doing that, I would add ultra important files like assignments, encrypted financial records and at least any photos which are improtant somewhere else, perhaps online.) Perhaps an external drive isn't easy to afford in your circumstances, but it would be better to start with some backup now rather than deal with whatever problems the laptop has. Worst case scenario, if you still have the other laptop you could store data there. Now if there's really nothing you care about on the laptop, then I guess you don't have to worry about backups but in the modern digital age, I would be a bit surprised by that. (16 years or so ago, some of my friends would semiregularly just format their whole hard disk and reinstall windows, but that was a different age.)

As to the hard disk, 3 years isn't very long, but it also isn't that short. A hard disk dying within 3 years definitely isn't that abnormal although probably more die in the first year than the 3rd year. About the noise, while it's somewhat understandable you may have never paid much attention before and while it's possible the recent problems have made you more sensitive to noises, the fact you notice it now does suggest it could be new.

As to the crash, none of what you've said leading up to the crash or the crash itself would be unexpected if there was a hard disk problem but nor does it intrisicly suggest one. (I'm presuming you didn't hear funny noises at the time since you would have said.) However, even without the other weirdness with the hard disk, while there are other reasons the operating system won't be found, I would suspect the most likely reason would be that the hard disk was temporarily missing due to problems.

As for "fix all" not appearing, it's mostly irrelevant whether or not you have an external drive. The most likely reason why it's not appearing is because you're using the wrong tool. Are you sure your hard disk is Seagate (including any made by those they purchased, like Samsung)? If your hard disk is Western Digital or Toshiba (or any of the brands they own), you should be using a tool from that manufacturer. (A search of the drives model number should tell you who made it.) Some tools won't even start if you don't have a drive from that manufacturer (this is when the prescense of a external drive may matter), but others may start but not offer all features (this will often also be the case if it starts because you have an external drive of theirs). I don't think Western Digital has a "fix all" in their tool (at least I don't remember one), but it's possible their extended test works in a different way. (I was under the impression these were generic SMART tests, but I've also seen some stuff suggesting WD's tool may reallocate sectors during the extended test.) I've never used Toshiba's tool so I don't know what it's like. At the very least, I would run all non write tests from the tool provided by your manufacturer. The best way to ensure reallocation of all sectors is normally to write zeroes for the whole disk but that will also erase everything. If the bad sectors don't have any data assigned, you could always use some tool which will erase all blank space.

Nil Einne (talk) 04:56, 19 October 2015 (UTC)