Need help with old CPU

Hello. I am new and hope I'm posting in the right spot.

Here's my situation: I have 3 computers in the house, but one is for my son JUST to get on the internet. It's an OLD Gateway computer that I never used. Recently I took his hard drive out and put another person's in there so I could transfer some of his files through my network onto my hard drive.

After finishing up, I put my son's harddrive (a maxtor 40gig) back into the cpu(the gateway) I took it from and tried to boot back up. Now suddenly the motherboard will not recognize it.

I know from past experiences that old motherboards have trouble recognizing Maxtor drives. But this one was fine until I took it out and put it back in for some reason. If I add a second hard drive(a 100gig western digital) to that SAME cable as a slave, it recognizes that one, but still doesn't recognize the Maxtor on the master cable.

So basically to answer a few questions. Yes I have the jumpers set right. Yes the cable works, and yes my motherboard does recognize a 100gig drive(WD) but not the 40gig(maxtor). I don't want to format the 100gig as it has a TON of stuff I need on there. The only extra HD I have right now is a maxtor, which already has windows on it and everything ready to go.

Any suggestions? Thanks so much!

Is it attached to the same connector on the cable as it was originally? All this slave/master stuff can sometimes be really irritating.

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