I have a Seagate Elite 9, 9GB, double-height 5.25" 50-pin SCSI HD and it's proven to be very reliable. It's a GIANT. I bought it years ago thinking it would fit in my 6100/60. Probably NOT. It took me a while to find an external SCSI case that would take it; I had a few but they were all too small, and only my Mac HD 20SC could give out enough juice to spin it up to full speed. So I had the drive sitting in the HD 20SC case with no cover on it. I overheated the power supply almost right away, which caused it to have some sort of fault protection kick in, so I took the metal lid off, covered it with nylon screen, and put a 12V DC fan on it. It sure looked cool but I bet I wasn't meeting specification in a few areas... That drive was LOUD when it was out in the open like that.
I went through 3 hard drives on that 6100 and finally I got the Elite 9 and it still works to this day. I also had a Sonnet Crescendo 240MHz G3 card burn up in that thing, within 90 days of purchase. I was still able to exchange for a new one, which that still works too.
I also had a Bigfoot drive (1/2 height 5.25") and it didn't last long, so I can second that they're not too reliable.