Chosen Solution
2008 iMac. Reboot gets 1/3rd completed, and system hangs. Restored OS. Booted with SHIFT to disable plugins etc. No luck. Shut down. Pulled the power cord for 30secs. Booted. Success. What’s still active and may be problematic when shut down, but still plugged in? Thx. Update (08/23/2018) This is a 20” unit. Thx. Update (08/23/2018) Yosemite machine. System refresh from hidden partition did not help, except a reboot after pulling power cord for a minute - then only once. Created a Yosemite install USB (fresh Yosemite from App Store/Purchased), then terminal shell create. Could NOT boot USB Installer “until” removal of power cord again, then success again, and fresh Yosemite install (first failed, but succeeded on 2nd try). Considered there might be a hard drive controller issue, so replaced with an SSD drive and Migrated user/data. Situation did NOT change; indicating possible motherboard issue.
First you do need to be careful on what SSD you put into this system! These older systems are only SATA II (3.0 Gb/s) so you need to make sure the SSD is able to run at this slower speed most don’t, Samsung 860 EVO is one that does. I would start off giving the system a good clean of the dust build up and given it’s age I’m sure the CPU & GPU are in need of fresh thermal past and the GPU will also need pads for the V-RAM chips.
