Has Mavericks gone all maverick on my 2010, 27" iMac? Long story, short: Replaced 'bad' HD with identical model (1TB), reloaded apps, + got external 2TB HD. Fan was running high, now machine won't boot in ANY mode.

Question

I have a mid-2010 iMac (27") 2.66 GHz Core i5 (750) with  4GB RAM that I bought second-hand almost three years ago.  It worked perfectly...until I 'upgraded' to Mavericks last Fall, & all my trouble began. At first, certain apps started getting kinda wobbly, especially Chrome which was really unstable. Eventually one day on boot-up I got the gray screen/apple logo/spinning wheel/loading bar--and then it would shut down.  I got apple care on the phone and we went through a laundry list of various tests and start-up attempts.  We reset PRAM, got me into safe mode and eventually hit a point where we were going to have to wipe the HD and start fresh--except I didn't have a back-up (shame on me!).  He told me 1) the error code that came up after running disk utitility (?) indicated no problem with HD and that it was software related; and, 2) to take it into Genius Bar, have them backup data and then I could call him back and we'd do the deed and then reinstall with backed-up files.  Tried but Geniuses said, "We don't do that" and told my hubby how to.  We bought the equipment and prepared to back-up using my son's MacBook Pro.  No glory.  Nada.
After much back-and-forth with 'geniuses' and guy on phone, we took it to MicroCenter.  Their diagnostics said HD is shot--get new one.  After pricing everything out we went with iFixit's instructions, and I ordered the identical (I thought) HD for Mac online from manufacturer.  We installed it, but had wee problem with LCD temp sensor and so fan ran like crazy.  I also bought a 2TB external back-up and did routine back-ups, so I should be good there.  I am aware of that third rail on the HD's with sensor but thought I could circumvent that and 'knew' it was really the LCD sensor that was the problem.  I was traveling for months all winter/spring so didn't have time to devote to a pesky fan noise.  HOWEVER, with increased use being home for several weeks, suddenly we are back to square one it seems.  One day the machine shut down and has not rebooted since--about three weeks ago now.  Haven't called apple care but tried to reset PRAM and tried to restart using thumb drive where I thought I had saved a copy of Mavericks--didn't work so don't know for sure if it's on there.  I opened machine back up and saw that LCD sensor wire was not connected and falling apart.  I ordered a new wire which installed tonight with no apparent problems, crossed my fingers and...NADA.  Same-o, same-o:  gray screen, apple, spinning wheel, loading bar...then it shuts down and goes to black.  I have tried to restart with all kinds of codes:  reset PRAM, safe mode, hold down option key, hold down Command+R...again, nothing. I even replaced the keyboard batteries. As the progress bar fills up near the end, I hear a little click and whirr, fan running softly, but then it just shuts down.  I should mention that the PRAM reset did actually start out like it was working--it shut down/restarted but then the daggone apple/gray scree/spinning wheel/progress bar would kick in again...and then shut down 5-10 minutes later.  Any ideas, folks?  Please help?

Answer 1

Your internal hard drive is not good.
Your Time Machine drive with your backups on it, if these backups were created using OS X Mavericks version of Time Machine, these will not be accessible by early OS X versions of Time Machine.
Plus, Time Machine stores data and nothing of the OS X system.
Plus, you want to save/retrieve your data that is still on your iMac's internal drive.
So, we can try to get your iMac back to running OS X 10.9 Mavericks.
The most easiest and direct way to do this is to just purchase another external 2 TB hard drive (preferably a fast FireWire 800 external hard drive)
You can use your OS X Snow Leopard install DVD to format this new hard drive as an OS X Extended Format with GUID partition scheme.
Create two partitions on the new external drive one that is big enough for OS X and all applications (say 500 Gbs) and the remaining partition for storage so you can manually copy all your important data ( and maybe your  Applications folder, too) from your Mac's internal drive (if it is accessible) to the large partition on this new drive.
Once the drive is formatted using the Disk Utility app that is on your OS X Snow Leopard DVD disc, you can then proceed to install OS X Snow Leopard to this new external drive and now giving you a bootable hard drive that will have a fairly fast, working OS X version your iMac can boot to, now.
Once OS X 10.6 Snow Leopard is install to this new external hard drive, you need to boot to this to get up and running.
Use OS X Software Update feature to get you back to OS X 10.6.8 Snow Leopard so you can regain use of The Mac App Store.
You have two options, now.
You still need to attempt to manually copy over all your important data and applications over to the large storage partition you created on this new external drive.
Once you have done this, you need to search for, purchase, download and install a data cloning app, like CarbonCopyCloner or SuperDuper to be able to clone your new hard drive's system back to your iMac's internal drive once you have resolved/replaced you iMac's Internal hard drive, again, if it is damaged.
If you still want to be able to access your Time Machine Backup drive, you can go ahead and try to re-download OS X 10.9 Mavericks ( you may need to use a Combo Update version from the Mac App Store, now as Mavericks is up to 10.9.5, now. You will upgrade install Mavericks over your new OS X Snow Leopard partition on the new external hard drive.
If, successful, you should be able to access your Time Machine backup at this point.
You will be running your iMac from this external drive until the internal drive is repaired/replaced.
The other idea is if you do not care about your Time Machine backups on the other drive, you can foresake the data on this drive and keep this new external drive , and your iMac,  running OS X 10.6.8 Snow Leopard and forget about upgrading to Mavericks, at all, since you had so many issues with it.