problems transferring data from external hard drive using Migration Assistant

I recently had to replace the hard drive on my 27" iMac (10.6.8) and am now trying to copy my data - basically what was on my user account - from an external hard drive onto my mac using Migration Assistant. The hard drive is plugged into a USB port and shows up on the desktop, but when launching Migration Assistant it does not show up in the "Select Your Disk" window - all I see is a spinning wheel. Any idea of what's going on?