I recently aquired a 2.4Gb Conner CFP2105S Hard Disk which I wanted to install in my Colour Classic (with LC 575 l/b). Because it's originally a PC drive, I've been trying to hook it up to the CC externally, using the SCSI case from my Apple CD 300. (So I can format and patition it, before transferring it into the internal bay.)
The first time I tried, everything seemed to go just fine, the drive spun up when the Mac booted, and I was able to detect it on the SCSI chain with the Lido 7 drive-formatting software. However, midway through formatting the drive, the process failed, and the hard drive began to spin up, spin down, spin up, etc. ad infinitum.
Now it does that all the time. When I first turn the drive on it doesn't spin, which I think is normal. When I then turn on the Mac it spins up, then makes a 'click CLUNK' sound before spinning down, and then the whole process repeats. If however, I switch the Mac on, and then switch it off immediately after the drive spins up, it remains spinning normally. (Turning the Mac back on resumes the spin up/down cycle.)
I'm pretty sure that this is just a bad drive, but I remeber having a similar problem with an Apple-branded 1Gb Quantum drive which I was trying to do the same thing with. I ended up throwing that drive away, but now I'm wondering if maybe there's more to it. I'm setting the drive to SCSI ID 1, and have tried playing around a bit with the Spin Delay/Spin Off jumpers to no avail.
Prior to either of these two drives I have sucessfully hooked up various other drives to my Macs using this CD Case, but to my SE/30 rather than the CC/575.
The Conner has the same problem installed internally in the CC, so I'm on the verge of binning it, but thought I might solicit a few opinions first...
It is a hardware malfunction. I think it is the hard drive controller (the chip that controls the read/write access directly on the platters through the head, and the sleep/standby function) I doubt you will be able to fix it unless you have the chip on hand and have very fine soldering skill.
Do you have another computer you can put the drive in and see if you get the same result? If the drive is indeed at fault, it should replicate most, if not all, the symptoms regardless of what computer it is in.
As well, check the jumpers on the drive and see what ID it's set to. Make sure that ID and the ID on the back of the external case match up.