Adding a second hard drive increases the system's power requirements by about 25W. With a 400W PSU this might be enough to tip it over the edge, depending on what else you have in there.
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As HWM says, try the new drive by itself set to master on IDE0 to see what happens. If there is no OS on this drive, use a boot floppy, then type C: when it boots to see if the drive can be read.