Don't use Seagate SATA drives in a G5.
They have a bad history of firmware conflicts with G5s.
Further, don't expect ease of use with over 2 TB drives in a G5.
The controller only recognizes 2.2 TB, so partitioning will be necessary IF the drive is properly recognized at all.
Stick with MAX. 2 TB Western Digital Blue and Black drives or Hitachi 7K drives.
These drives are known to work well in a G5, even in the case of a SATA rev. 3 drive.
WD drives will require a jumper on pins 5 and 6 to limit data rate to work with a G5, but are otherwise trouble free.
Hitachi 7k SATA rev. 3 drives (thus far) do not require a jumper to work in a G5.
AVOID all green, 5200/5400 RPM drives, regardless of maker or capacity.