Yosemite hangs on startup

i posted somewhere else on site (my first ever) , realized should in forum. sorry repeat.

hi folks, know came several months ago, downloaded yosemite , having major problems. have 2 year old imac, loaded. had mtn lion running (not mavericks). can't boot up. download progressed, auto restart hangs after 25% loaded. no amount of rebooting works. tried holding down many different keys using other people suggestions, nothing works. can't @ safe boot. can't @ disk utilities, nothing. have full backup few week ago had 1 file on desktop had lots of recent work not backed , hoping not lose this. can't access external anyway cuz holding down option key during startup isn't getting me anything. starting panic.

appreciated. , btw yes did wait many hours after first yosemite download , got endless spinning colour wheel of doom.

update: unplugging mac , waiting 10-15 minutes able (1) hold shift @ startup , @ disk utilities, can access time machine on external drive (but erase of current hd including file i'm hoping save, , (2) hold shift key , login screen safe boot, when enter mac password (yes i'm sure it's correct) , click forward arrow hangs, spinning. never goes safe mode. tried command+r start disabled wifi , restarted, no help.

suggestions?

 

success, tho hard come by. managed clone of hd onto second external, used time machine reload old copy of including old mtn lion system onto mac , ran again. clone of hd had files not backed on tm hoping not lose. well, thinking long , hard before down loan yosemite again.

 

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