Is there a system on it now? Will it read any other disks?
If not, it is probably not the disks and more than likely the drive itself. If it does see the disks when booted by whatever is on the hard drive, then it is probably a weird issue with a bad PRAM battery.
Laptop: Late 2006 2ghz Black, Pismo 400, 1400c x 3
Duo: 230, 280c, 2300c x 3, mini dock x 2, Duo Dock II
Desktop: LC, LC 475, G4 Cube, IIfx, Umax C600, PM 8600, G4 1.4ghz DA, Mac Mini 2011 i7 2.5ghz