Currently Being ModeratedMar 29, 2013 9:11 AM (in response to iMac are the best)
You should be able to hold the "Option" or "alt" key after you press the power button, and a list of bootable drives will appear. Under this list of drives, if you did it correctly, you will see your bootable drive. Note that if this drive is USB 2 or even USB 3, you will see a considderable slow-down in speed, but you will still atleast be able to use the laptop during that time.
Currently Being ModeratedMar 29, 2013 1:11 PM (in response to iMac are the best)
Thanks for the responses, just checked, it booted up fine and obviously ran slower than expected. I just need to confim that this is ok to do for a few days whilst the iMac isn't around and will cause no problems when I copy it back to the new blank HD.
Also will time machine run in the same way (on another seperate drive) whilst I am doing this, so I have multiple copies incase of any issue.