Fresh install of both El Capitan and Yosemite on the hard drive with two partitions produce the same exact result...not completing the boot process. Can't do Internet recovery and the recovery partition does the same thing...not completing the boot process. Had to install both OS via firewire connected to my MBP. Then, those third party websites offering the Apple Hardware Test turned out to be empty folders with no diagnostic software in core services. Fsck -fy shows okay. Wish to know what's happening when booting El Capitan, and the process bar gets to the middle...then makes this jump (the progress bar...sometimes the bar turns white) then completes the booting...I watched it on my laptop, and knowing this would provide the solution as to what's going on with the iMac. Why it's not making the jump...and jump to what?