Wikipedia:Reference desk/Archives/Computing/2021 March 19

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March 19

Drive letters relative to the OS in a dual boot

Suppose that you have both Windows 8.1 and Windows 10 installed on the same computer, for example. Then, Windows 8.1 will think (from its own perspective) that it is drive letter C and Windows 10 drive letter D, while the reverse happens with Windows 10, thinking (again from its own perspective) that it is drive letter C and Windows 8.1 drive letter D. Why does this happen, with drive letters being relative to the OS in a dual boot? GeoffreyT2000 (talk) 23:31, 19 March 2021 (UTC)

Drive letters aren't an inherent part of the drive; they're unique to Windows. Thus, Windows stores these values in system files, not on the drive. Though Windows will support any letter as the system drive, a lot of programs are written to assume the system drive is C: so Windows defaults to that. You can change a drive's letter in Disk Management, but that's not a particularly good idea to do for your system drive, since references to locations to it will not necessarily be updated and you will almost certainly break a lot of things. TheMrP (talk) 06:57, 20 March 2021 (UTC)

The same happens on Linux. The exact same physical drive can end up as, for example, /dev/sda or /dev/sdb depending on which order the system finds the drives in on boot-up. The only operating system I can think of that provides unique identifiers to individual disks rather than the devices they are accessed through is AmigaOS. JIP | Talk 21:28, 20 March 2021 (UTC)

Windows and Linux both refer to disks, and partitions, by UUIDs (which do not change if the physical disk is wired to a different port). At least in Linux (and surely in Windows, but I don't know how) it is possible (and to my mind much better) to mount volumes by their UUID. There are no /dev/sd? entries in my fstab. -- Finlay McWalter··–·Talk 22:04, 20 March 2021 (UTC)

There are UUIDs, /dev/sd? entries and /dev/mapper entries in my fstab. The /dev/mapper entries are for all my HD partitions except the boot partitions, as I use LVM to allow for a partition to cover multiple physical disks. The /dev/sd? entries are for the computer's memory card reader. I figure that if I used UUIDs there I would have to insert the exact same card each time, which would be rather difficult when using multiple memory cards. JIP | Talk 00:19, 21 March 2021 (UTC)

UUIDs are pretty horrible (for instance "fed1734e-bc11-4055-a65d-6423a7d9bcd4"). Generally I prefer to set labels on each filesystem and mount by "LABEL=Root" in fstab. Martin of Sheffield (talk) 04:40, 21 March 2021 (UTC)