Here's one thing you could try, if you think it could possibly be a bad hard drive: try taking the hard drive from your old pc, and running it in the newer one. If it's the hard drive, you may be able to boot with the old hard drive, and then possibly get the newer hard drive to spin up as a secondary drive, and then copy any important files off of that drive onto CDs or DVDs or perhaps another hard drive..
As for the size limitations, it would be worth doing some searching around on the web for any info specific to your motherboard (if custom built) or desktop machine model number (if from an OEM like Dell, IBM, Compaq, HP, Gateway). You may find, as cdogg mentioned, that your old machine will need a BIOS flash in order to read even the slighly larger hard drive. At the time those were likely manufactured, a 15 gig hard drive was a HUGE increase from a 6 gig hard drive, of course! So, it is very possible. It usually depends upon the particular motherboard as to how large a hard drive, how much memory, etc, a system can handle. You could have the exact same processor in 2 different motherboards, and have totally different results with hard drive and memory limits.
Another option would be to try swapping out the IDE cables in one of the machines, if you know where an extra is. Also, you could try hooking up the 15 gigabyte hard drive on your older PC as a secondary drive, and your 6 gig drive as the primary, and see if it will still boot into Windows 2000, and then possibly see the 15 gig as an extra hard drive. I have had a hard drive or two before that couldn't be booted, but could be seen for extra storage - at least enough to back the data off of those drives for recovery.
And, of course, some easily overlookable thoughts with hard drives, when you're moving stuff around, that sounds simple and impossible to forget, but it's easy to do:
1. Make sure you did connect both the IDE and power cable to the hard drive, and the other end of the IDE cable IS securely connected to the motherboard/mainboard. It's super easy to forget to connect one wire, when you're frustrated, and/or moving different parts around between machines.
2. Check the jumper settings on each hard drive. If you want to use one as the primary, make sure the jumper is set to "master", and the secondary is set to "slave." This is, of course, assuming you put them on the same IDE cable. If you use separate cables, with no other drives attached on each cable, each jumper should be set to either:
1. Master
2. Master with no slave attached (if this option is available)
3. Or, you can just set it to CS (cable select) at times. But, when trouble shooting, I'd stick with setting each drive exactly to what is desired, at least at first.
hth
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"If to err is human, then I must be some kind of human!" -Me