iMac won't boot

[SaSaSushi]

Actually, if you know how to hold a screwdriver, I'd suggest something completely different.

You might get something like this:
http://www.amazon.com/dp/B003VKTJGW...UTF8&colid=R75PP4I2A0BE&coliid=I3DOKZ31SP7539

Well, heck, if you're going to be going with a 2.5" drive then why not get the Inateck FEU3NS-1EUASP enclosure.

No screwdrivers or any other tools required and it's only $18.

It's really a nice little enclosure. It's also bootable

See my review of it here.

However, although you can use hard disks, I think the main reason to get a 2.5" UASP enclosures would be to get max speeds with an SSD and I know you happen to be looking for a means of backup.

If the WD is at a good price, it'd be fine or just get any nice USB 3 3.5" enclosure or cradle with boot support you can find with some good reviews and then pop in your own HD to save a few shekels.

 

[krodger]

It Worked!!!

Have you tried SMC and/or PRAM reset?

SMC Reset

1. Shut down the computer.
2. Unplug the computer's power cord and disconnect all attached devices.
3. Wait fifteen seconds.
4. Attach the computer's power cord.
5. Wait five seconds, then press the power button to turn on the computer.

Reset PRAM

1. Shut down the computer.
2. Locate the following keys on the keyboard: Command, Option, P and R.
   You will need to hold these keys down simultaneously in step 4.
3. Turn on the computer.
4. Press and hold the Command-Option-P-R keys. You must press this key combination before the gray screen appears.
5. Hold the keys down until the computer restarts and you hear the startup sound for the second time.
6. Release the keys.

This problem with my relatively new iMac was getting me really frustrated. I had actually thought my problem was a failing hard drive, for about the last week the computer was acting a "bit goofy" - programs wouldn't launch at all, or if they did launch they were unresponsive (Aperture, iTunes, Preview). A couple times, I simply had a grey/silver screen that was unresponsive. When I rebooted the computer appeared to operate normally. When Aperture started to act up, I was finally able to get it to launch, but then it had no access to any of my photo files held on an external 1TB drive. The drive was showing, but I couldn't open it up or eject it. I unplugged the drive, and plugged it back in (USB3) and Aperture seemed to work fine. Then two days later, I was presented with a white screen, nothing else - no Apple logo - nothing. After rebooting, that is what came up once again.
I made an appointment with the Genius Bar at our local Apple store, but before I went I decided to try the SMC reset - what have I got to lose. And it worked. Hopefully it stays that way! Never had this problem at all on my other Intel Mac in over 7 years.