SCSI drives almost never go bad. Compared to IDE/SATA, they were significantly better built and had lower failure rates. I still have a few 15k RPM Cheetahs that still work, last I checked :).
In general, it is surprising how many old hard drives still work. I have a good number of old SCSI drives (even Seagate!) that still work 40 years on. The ST225 in one of my XTs still works great as well. I actually have a ST238R still new in box, and I'll be curious to see if it can spin up some day. I suspect the bearings might be a stuck after all of these years!
They can be baked to re-blend the lubricants and free up bearings. 200F for a couple hours, as i recall.
The SCSI drive in my Mac SE is older than me and it still works. Delightfully clicky, in fact louder than the floppy drive!