Fail to boot up after downloading Yosemite

[ArkyMac]

Hi folks, my first post. I know this came up several months ago, but I just downloaded Yosemite and having major problems. Have 2 year old iMac. Had Mtn Lion running (not Mavericks). Now can't boot up. The download progressed, but then auto restart just hangs after about 25% loaded. No amount of rebooting works, nor does waiting many hours make a difference. Tried holding down many different keys while rebooting using other people suggestions, no luck. What I can do is boot with Option key and see Disk Utilities, and boot with Shift key and get to the login screen for Safe Boot but when I enter my Mac password the dreaded colour wheel of doom just spins forever, I never get in to Safe Boot. Seems like the Yosemite download has disabled or changed my keychain password. I do have Time Machine backup to about a month ago, but have one Word file on current desktop that was recently working on and hate to lose it. So I can restore all with Time Machine from my external, but will lose everything on current HD. Starting to panic.

I tried Command+R start then disabled wifi and restarted, no help. Also tried Option+Command+R which is supposed to boot from internet and not internal drive, but this also takes me to Disk Utilities and I can't see how to fix problem from there.

UPDATE: today i was able to get into Internet Recovery, the default option here is to reload the original OS to the mac,which would be Mtn Lion and that would be great, but when it goes to verify this it tells methat it can't write the system to the disk. I can reload Yosemite and have done so twice,but no improvement.
Any suggestions much appreciated.

 

[Raz0rEdge]

Welcome to Mac-Forums..

Working through the steps, you went to Mac App Store and began the Yosemite download. Did the download finish and then begin the Yosemite installation? It is at this time that a restart would have been attempted..

I don't imagine upgrading from Mountain Lion to Yosemite as opposed to Mavericks to Yosemite should be an issue, but it looks like the Yosemite installer has at least setup the recovery console to re-install Yosemite in the future which is why you are seeing the option there, otherwise it would still be pointing to Mountain Lion..

If you can get in on Safe mode, you can save the documents you want, otherwise your only recourse might be to go through the recovery partition, erase and re-install Yosemite and then bring back your data through the Time Machine backup..

 

[ArkyMac]

Thanks for the reply. You are correct, the hang after the download and installation. And for reasons unknown I can't get safe boot to work. Frustrating, cuz all I need is one minute access to my desktop. I don't want to reload Yosemite, I want out of that system, have downloaded it 3 times now all with the same results. I'm about to give up on my files and restore using my month old time machine from external. I'm of the impression this will reload everything from my old computer including the previous system, is this not correct? If so then I won't have to worry about dealing with Yosemite.I have a second external (one without the time machine file) and tried using OS X Utilities "Restore" to copy the entire current HD to this second external. Thinking I might be able to get at desktop files from this after I reloaded Time Machine on the imac. But this action was refused.
Please let me know if you time machine reloads the previous system. Thanks.
[Canadian Patriot fan]

 

[ArkyMac]

Update: Success. But was hard to come by. In the end I managed to get OS X Utilities to run a copy of my current HD onto a second external (not the one with my Time Machine) despite my first attempts to do this failing. Using a different Mac I played around with the read-write functions on the external and this seemed to make the difference. So copied my HD, then went to my month old TM and restored all my files and my old (Mtn Lion) system on the iMac. When this was done it ran perfectly, and the few files not backed up on TM were there on the clone of the HD that I made on 2nd external. So all ended well after two days of ****. Will be thinking a long time before I try to download Yosemite again.