OS 10.5 - G5 stopped booting to OSX Leopard, HD not recognized as bootable

I'm calling my problem an OSX issue, but may be a hardware issue. I installed OSX Leopard in February 2009, and it ran good until yesterday (april 20, 2009). When I booted the MAC yesterday, it showed a blinking folder with a question mark inside. I inserted my OSX install disk and ran the Disk Utility. It recognized my hard drive, but said it was unformatted, Also the "Verify" and "repair" buttons were grayed-out. I pulled out the hard drive and connected to an external hard drive enclosure. I was able to read the drive on a different MAC, and recover my data. The hard drive is still good, it is literally only 3 months old. Why would my MAC stop recognizing the hard bootable drive? This is the second time this has happened to me in the past 6 months, the previous time it happened with an older hard drive. BTW, both "bad" hard drives work perfect on Windows PCs.