Apple Hardware Test won't complete. It seems to stop after about 20 minutes. I can still move the mouse, but it's very jerky. The time shown next to "Total Time Testing" has not updated in several hours. I suspect there's an issue with the hard drive. User tried to install upgrade to Yosemite and it failed with an error about some type of disk issue (sorry, don't have the exact error, but searching on that error indicates a disk problem). Disk Utility from the installation utility and with the Mac attached to another via Target Disk Mode reports no issues. Can't boot my copy of Disk Warrior. Screen stays white with Apple logo. Might be that my copy is too old for this i5 iMac. Super Duper almost makes a complete copy of the disk to another device, can't find exact error right now. I was hoping that the Apple Hardware Test would tell me the answer, but it won't finish. I've run the native test and used the Option - D to get it to download the test. I hate to open it up and replace the drive just to see if it makes a difference. I'd like a better diagnosis. Any thoughts?