Physically disconnect the dodgy drive, & change 1st boot drive to the SATA 500Gb in the BIOS. It's probably already setup this way, but confirm all the same. See what difference this makes.
Reconnect dodgy drive & backup its data to another one of your drives if necessary. If you find the startup & continuous activity problems again, it may be the drive giving up the ghost. Attempt a reformat, but if the problems persist disconnect it for good. G
P.S. Another thing you can try is changing the IDE drives' statuses from slave to master via the jumpers & vice-versa, or experiment with cable-select jumper settings