In fact, the only device which will try to spontaneously grab the bus is the Macintosh itself. The only exception to this is if the Mac asks one drive for the some particular data and the drive has to move the heads there and the Mac has to wait, then the Mac will allow the other device to cut in while it waits for the heads to move. I reinstalled the system software, but the problem persists. If you didn't do a clean installation, the problem could still be hiding in your System Folder. If it goes away, you've got a conflict.
More memory is often the solution to a lot of different things.
With Umax, you use Hard Disk Toolkit to do that. Take it out and see what happens. You have to download the specific version which has 128-bit encryption, and you'll know you've found it when you have to fill out an online form before you can download it. The current beta of Communicator 4.5 doesn't support 128-bit encryption. The formula is one-half K for each 32 megs.