Having worked with dozens of these smaller capacity drives, here's a bit of insight into the brands and the like:
Conner: The 40MB Conner drive was very common in Classics. I've also seen it used in LCs, but the Quantum LPS is much more common there. I've had very good luck with Conners; only one has failed. They weren't designed to be a performance drive; if memory serves me correct, they score lower on benchmarks than the Quantums from the same time period (see below). It's also tough to rig up an LED to these if you're putting them into an SE. Still, I prefer these drives to their Quantum counterparts in 2013 and am making an effort to look for scrapped Classics simply to pick up these drives.
Quantum: There were many different Quantum drives used by Apple and other companies from the late 80s/early 90s. Here's a breakdown of the different drives and my observations:
LPS Series: These were probably the best drives available when they came out, as they were quiet, fast, and reliable. That is, until they got to be about 15-20 years old. The majority of mine have died. No capacity seems more reliable than another. They don't seem to play nice with early Classics for some reason. I had one in a Classic with a Revision A analog board in 2000 which only spun up half the time, if that. When I put it into an SE a few years later, it worked absolutely fine. These drives were very common in LCs, IIsis, and later IIcis.
ELS Series: These are noisier than the LPS drives and even less reliable. Most of these are 80MB or larger and seemed to come out around 1993-ish. I think I'm down to two of these in working condition. I've seen these quite often in Color Classics.
Anything older than 1990: Hit or miss for 3.5" drives. The 40MB drives seem to hold up pretty well. The 80MBs are notoriously unreliable. I have yet to find a good one; these were even bad when they hit the 10 year mark. The 5.25" drives weren't bad. You can find these drives in some SEs and SE/30s, as well as some of the earlier Mac IIs with 3.5" drives.
Miniscribe: These are the slow, noisy, often trouble-prone 20MB drives in SEs. I've actually found the good ones will go on for a long time; many of my current spare drives are working MiniScribes.
IBM: I only have one of their 3.5" SCSI drives, but it does seem quite good. I think I'll be looking for a few more as my Quantums continue to succumb to Quantumitis.
