I unfortunately found out that not all SATA drives are created equal. I recently bought a DP 2.7GHz G5, then shortly after bought a WD400GB WD400KD off of ebay (new, but sold as is). It would only work for small files. Copying anything larger than 1 or 2 GB would cause the computer freeze. Who buys a 400GB hard drive to copy small files???? Thought it was a bad drive, but researching shows this is a very common problem for G5s and SATA drives. I saw this a xlr8yourmac.com....I had to buy a sata pci card to use the drive!!!!!

Some SATA drives have to have a certain "feature" (Spread Spectrum Clocking) disabled by a firmware flash or a jumper to work in the G5

A workaround is to use this tool:
http://www.seagate.com/support/kb/disc/faq/ssc.html
on a PC and change the drive's feature

PS.: Read more here:
http://discussions.apple.com/thread.jspa?messageID=595352