I get errors on the stock 250GB HD that came with my G5 2.7GHz Dual. It seems to have errors on it, but doesn't let me fix it as the Repair Disk button is greyed out. Anyone seen this before?
Use the Install disk. Before you choose the volume to install to, go to the Menu>Utilities. Launch Disk Utility, then chose Repair Disk. Reboot to OS when finished.
Because the boot disk cannot be repaired when you are booted from it. As noted by bigboy99, boot from your installer DVD and use Disk Utility to repair the drive from there.