Several months ago, I reached capacity with my current RAID 5 array (3x 4tb drives). I considered adding another 4tb drive to the array, but there were a couple problems. First, RAID 5 is an inherently risky setup. While theoretically, it can handle the loss of a single drive without issue (single fault tolerance), the problem arises during the rebuild. In order to recover from the loss of a drive in RAID 5, it's necessary to replace the faulty drive and then to recontruct the lost drive by setting each bit Ak with the result of Bk XOR Ck XOR .... In doing so, it necessary to read every bit in the remaining drives, and the possibility of encountering an issue with one of the other drives is high. (There is reason to believe that the claims of a 50+% chance of a URE when rebuilding a RAID 5 array are sensational, but there is still a non-trivial risk of a second drive failure that grows with the number of bits and number of drives.) [Link to that article claiming URE is more likely than not, and also criticism / refutation]
https://news.ycombinator.com/item?id=8306499
https://smbitjournal.com/2012/05/when-no-redundancy-is-more-reliable/
https://heremystuff.wordpress.com/2020/08/25/the-case-of-the-12tb-ure/
But I've never been particularly risk-averse. What better way to come to terms with the impermanence of all things than to lose 8 terabytes of data? The perhaps greater reason was an issue of practicality. My hardware was at its limits. The motherboard in my home server only had 4 SATA ports which were all in use (one SSD, three hard drives); the semi-modular PSU had a similar problem; and the chassis was overloaded with the 4 drives I already had, the third hard drive sitting on the floor of the case. So, rather than adding a PSIe card with extra SATA ports and replacing my PSU and case for at most a few hundred dollars, I elected instead to buy three new hard drives for $200 each. (Actually four, but we'll get to that later.) I'd seen 14TB WD Red Plus drives on Amazon for about $205 each, about $50 cheaper than they had been, and nearly bought them. But as I was steeling myself to spend $800 to avoid forcible adoption of the digital konmari method, the price hopped back up to $250+ each, and I quickly resigned myself to my minimalist fate.
-- Western Digital 14TB WD Red Plus NAS Internal Hard Drive HDD - 7200 RPM, SATA 6 GB/s, CMR, 512 MB Cache, 3.5" - WD140EFGX