well with the MAC is what i'm not as good with, but hardware is another thing. it sounds to me that the drive is bad, bad sectors, firmware, etc. but you do have a couple options to isolate the problem if the hard drive is in fact defective:
Do you have another computer you can try, if it affects another computer the same way, probably tells you something.
There is a program called tech tool that i've used in the past the check the drive. there is some info here:
http://support.apple.com/kb/HT2648
Here is another utility that is free for the first 5 times you try it or something then you must pay:
http://www.volitans-software.com/smart_utility.php
But one of my favorites is Drive Fitness Test (DFT) which is free, you create a bootable CD from the image file download in the link:
http://www.hitachigst.com/support/downloads/#DFT
I think this is one of the better ones out there to work with, easy to use and understand.
Or this option:
http://www.grc.com/spinrite.htm
or this one:http://www.seagate.com/www/en-us/support/downloads/seatools
These last 3 im not sure about compatability with a MAC, which is why i asked about PC availability.