Installed new HDD, now won't boot when old drive connected

[BOBL273]

Hello,
I instaled two new 500GB drives in my 6-year old PC. Installed XP w/o anyproblems. However, I need to copy my old files from my old HDD to the new HDD, but the PC will not boot when the old drive is connected to the MB.
All drives are SATA. Any help would be great! Bob

 

[AntiZig]

If it's not a hardware problem (AHCI vs IDE is pretty common)

does your old drive still have the OS on it? I think it might be getting confused as to which drive to load OS from, check your bios.

Also, you say it will not boot, does it refuse to even POST? or does it not find boot ini? cannot load windows?

check that your PSU can support that many HDDs