This may be a problem related to SATA speed. PPC G5's were set up to run SATA 1.5 GB/sec. The new WD drives are made to run SATA 6 GB/sec. The PPC may not be able to "downgrade" so that it can recognize and run the faster drive at a slower rate. In some cases, you can put a jumper on the drive to "slow it down"; not sure on this, you may find something at the WD website.
Hi, Dennis is correct.
The G5s can often negoiate a later SATA version drive down to SATA 1.5 Gb/s If it first finds a real SATA 1.5 Gb/s drive in the first position.
Just keep the new drive in the second position, there should be no noticeable difference in operation.