I just posted this somewhere else on this site (my first ever) and then realized it should probably be in this forum. Sorry for the repeat. Hi folks, I know this came up several months ago, but I just downloaded Yosemite and having major problems. Have 2 year old iMac, fully loaded. Had Mtn Lion running (not Mavericks). Can't boot up. The download progressed, but then auto restart just hangs after about 25% loaded. No amount of rebooting works. Tried holding down many different keys using other people suggestions, nothing works. Can't get at Safe Boot. Can't get at Disk Utilities, nothing. I have full backup to a few week ago but had one file on desktop that had lots of recent work not backed up and hoping not to lose this. But can't access external anyway cuz holding down Option key during startup isn't getting me anything. Starting to panic. Any help would be much appreciated. And BTW yes I did wait many hours after first Yosemite download and just got the endless spinning colour wheel of doom. Update: By unplugging the mac and waiting 10-15 minutes I am able to (1) hold Shift at startup and get at Disk Utilities, so can access my Time Machine on my external drive (but this would erase all of current HD including the file I'm hoping to save, and (2) hold Shift key and get to login screen for Safe Boot, but when i enter my mac password (yes I'm sure it's correct) and then click forward arrow it just hangs, spinning. Never goes into safe mode. I tried Command+R start then disabled wifi and restarted, no help. Any suggestions?