are laptop hard drives interchangable the way desktop hard drives are? (i.e. an IBM Thinkpad working with a non-IBM hard drive) or are they proprietary?

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interchangable

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Well, "sorta" interchangeable. "Generally," interchangeable.

I do have one drive that will fit my current laptop, as well as one of the older ones, that just plain won't detect and operate. These are both Thinkpads, and there is no option in bios for manually entering drive parameters.

This very same drive, though, will operate just fine, with a cable adaptor, on my desktop PC. The laptop that this drive came from is completely dead.

You probably should try and find the support pages for your laptop, and see if there is an approved list of compatable drives, and most important---make sure it is not too tall, or thick. One of my old drives, out of my old 380XD Thinkpad, is too thick to fit my newer one.