Hi, I have been working on getting my iMac to boot with no success. Initially the symptom was slow operation. This coincides with an attempt to upgrade the OS to 10.13. I was able to revert to the previous OS from a time machine back up, but the problem of being slow, snowballed at that point. Eventually got to where on reboot, I would get the white screen with no progress at all. I diagnosed this as a bad hard drive, which I had previously replaced a little over 2 years ago, 2TB Seagate SSHD. At that point I thought, "no problem, hard drive upgrade". I upgraded the opti drive to a 500 GB SSD so I could load/ run OS X separately from the new WD 2TB hard drive. To get started, i left the HDD disconnected and only connected the SSD. After several failed attempts of loading OS X onto the SSD, mainly through start-ip manager disk utility then tried at the command prompt. The system would get stuck on progress bar and would not finish. Could not get it done. So I purchased another SSD, installed into external bay. Was able to load a new copy of Sierra onto it using my MBP. Using start-up manager, I was able to start my MBP with this external drive. Then I connect it to my iMac, use start-up manager to select but still am not able to boot. Would get stuck on progress bar at about 75%. Then I used disk utility to copy the OS from my external drive to the internal SSD. I was able to verify that the amount of space used was similar about 13.2 GB on both drives. Tried to reboot, and get the progress bar that gets to about 75%. Next I reboot in Single User Mode. There is definitely an issue, because using /fsck command, get the disk modified. But on running /fsck a second time, volume is OK. So seems to be a single problem. I tried to reboot, but again got stuck in progress bar. Got into single user mode again, and ran /fsck. Same thing, single issue found. This time I "exit" to continue booting, but again, progress bar, this time no progress at all. It seems the problem is not the hard drive but something else. I need your help to troubleshoot. I am not that familiar with Single User Mode, but willing to learn. Thanks for your help in advance.