iMac G5 SATA hard drive keeps disappearing #

Same symptom, different causes:

Question mark on boot. Partition/format drive, run hardware tests (passed), install OS X - OK
After a week or two, question mark returns. Replace SATA cable - OK
After another week or two, question mark returns. Hard drive has failed (verified on another Mac) - install new hard drive and repeat step 1 - OK
After yet another week or two, question mark returns. Set jumper on hard drive to limit feature set to SATA revision 1.0 (SATA 1.5 Gbit/s) - OK (over ~~a month~~ 23 months and counting!)

See also:

Why will only certain SATA data cables work?
iMac G5 Hard Drive Upgrade--Jumper settings?
Mac Drive Upgrades/Compatibility Database
Seagate's Barracuda 7200.9 Serial ATA Installation Guide, Publication Number: 100390002, Rev. B, January 2006 - Your SATA-equipped motherboard or host adapter may use a chipset that does not support SATA speed autonegotiation or other 3 Gbit feature. Install a jumper as shown in the following illustration to limit the data transfer rate to 1.5 Gbits per second and enhance compatibility with some computers.
SATA 1.5 Gbit/s and SATA 3 Gbit/s - To prevent interoperability problems that could occur when next generation SATA drives are installed on motherboards with legacy standard SATA 1.5 Gbit/s motherboard host controllers, many manufacturers have made it easy to switch those newer drives to the previous standard's mode. For example, Seagate/Maxtor has added a user-accessible jumper-switch, known as the Force 150, to enable the drive to be switched between 1.5 Gbit/s and 3 Gbit/s operation.

/mac | Dec 25, 2011