Yosemite hangs on startup

[jack.brink]

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?

 

[jack.brink]

Success, tho was hard to come by. Managed to get a clone of my HD onto a second external, then used Time Machine to reload slightly old copy of everything including my old Mtn Lion system onto Mac and it ran properly again. The clone of HD had the files not backed up on TM that I was hoping not to lose. So all is well, but will be thinking long and hard before I down loan Yosemite again.

 

[JButlin]

Yosemite Insanity

Hi folks, I'm also new to this forum (and relatively new to mac OS). I'm having the same sort of problem - I've got 3 imacs, which are around 2 years old, all of which are getting stuck with a black progress bar underneath the apple logo at 50%.
I've tried resetting NVRAM, running the disk utility and repairing disk permissions but so far nothing has got me any further. For some reason, none of them will boot in safe mode, no matter how many times I try to reboot holding down the shift key.

I suspect all 3 imacs have been suddenly turned off at the plug, or we've had electrical problems at the school over night.

If anyone has any suggestions, I would appreciate it!

thanks

 

[jack.brink]

Sounds exactly what happened to me, and the same inability to effect many suggested solutions. First question before going any further - do you have backups of these machines on an external drive using Time Machine?

 

[JButlin]

thanks for your reply! Sadly, no time machine backups - but it isn't too critical as all the data is stored on the network.