Hard Drive not recognized

I have a Dell laptop with four drives [ each one in its separate caddy] all the drives worked a couple of months ago. I usually have the Win 2000 Pro drive installed and it works fine but when I install my 98SE drive it is not recognized, my 95 drive is but my backup copy of my 2000 Pro is not. What would cause the 2 drives to suddenly go bad while stored in a box with the others? When they are installed and I try to boot I get a message that drive is not found. Any ideas? They drives are about 3 1/2 years old and are 40 gig 5400 rpm Toshiba's. I have tried switching caddies but to no avail.

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Does your Dell have another bootable device (CD/Floppy)? For testing purposes.

Can you supply the actual model numbers of the failed drives?