Yosemite Bootup Issues

Using a late 2012 iMac running version 10.10.2, upgraded to Yosemite around November last year. I'm using the 3 terabyte "fusion drive" with the 120 SSD built in. For the past month or so, my computer has been running "slower" than it used to. More accurately, it would freeze certain processes, such as iTunes playing music or a web browser, for anywhere between a split second and several seconds, without slowing down other more basic processes being run, such as 4 corners, mouse control, and dock, and then resume back to normal shortly after.

Today, a freeze started as normal, but never stopped. After waiting for 2 minutes or so, I held the power button down long enough for the computer to turn off. I then booted up the computer successfully, selected a user, and typed in the password. The computer got stuck loading up the user, after waiting another 2 minutes, I shut the computer down and tried again. Ever since then, The boot up never gets past the halfway point on the loading screen before shutting down on it's own, never reaching the user page.

I googled the problem and tried what seemed to be the most obvious solution, disk utility. Holding down command R while booting successfully took me to the recovery drive, where I tried to verify and repair my hard drive. The available drives seems to be the following 6:

Macintosh HD

Macintosh HD

2 TB External Raid Media

iomega HDD

disk2

OS X Base System

With each even system being a subprocess of the odd system above it, as evidenced by the following picture.

[IMG]http://i.imgur.com/sKn3iTm.jpg[/IMG]

The 2 TB External Raid Media/iomega HDD is my external hard drive, which contains a 2 month old backup I'm hoping to use as a worst case scenario if I can't recover the existing files. I believe, but I'm not sure, that disk2 is the recovery drive, and thus not to be messed with, which leaves the two macintosh HDs, or HD1 and HD2.

HD1 only has the "first aid" and "partition" tabs, while HD2 has "first aid", "erase", and "restore".

Verifying HD1 revealed a problem, instructing me to repair it. Upon finishing the repair, I received the following error message:

"Disk Utility can’t repair this disk. Back up as many of your files as possible, reformat the disk, and restore your backed-up files."

Distraught, and after a bit more googling of that particular error message, I found a forum where someone claimed repeating the action over and over again eventually worked. So, on the 4th try of repairing HD1, it repaired without any error messages, bringing my hope up. Successfully repairing HD1 seemed to "mount" HD2 to HD1, and bring up the correct information about the size of the data stored on the disk, as well as the total number of files, which were previously missing.

Attempting to verify HD2 brought up the following error messages, and unmounted HD2 somehow.

[IMG]http://i.imgur.com/1osMqhf.jpg[/IMG]

Despite the clear instructions to repair the disk, the repair button for HD2 is greyed out, and mousing over it reveals that it's not available because "the selected disk can't be written to"

I then tried to boot in safe mode, holding down shift, which didn't work. Next I tried entering single user mode by holding down command S, which did work, and using the fsck -fy command, which brought me the following screen.

[IMG]http://i.imgur.com/Bpwrz1c.jpg[/IMG]

Apologies for the low quality, but I can't think of a better way of relaying the information than the subpar image outside of manually transcribing the very long error message, which I will do if it might help. The main portion of the error that applies seems to be :

checking catalog file

corestoragegroup: completeIOrequest - error 0xe00082CA detected for LVG "Macintosh HD" "Many random looking numbers:

Disk2: I/O error

The volume macintosh HD could not be verified completely.

I have no further ideas on what to try. Does anyone have any ideas on how to fix the drive before I reformat it and restore from a 2 month old backup?