Drive missing

I recently had to replace a users pwr supply in a Dell dimension 4100. While waiting for it to come in, I connected the hard drives in a GX260, as slaves to allow the user to retrieve some data off of each drive. Since I reinstalled the HD’s intothe system, I am unable to see the slave drive. Including in the BIOS. I’ve replaced IDE cables and switched around pwr cables. Any explanation as to how or why this may have happened? Any suggestions as to how I can fix this.