External HDD Keeps Hanging Computer

I recently upgraded my Macbook's HDD and cased the original 60GB one. I formatted it with Disk Utility to an MS-DOS file system and used it, switching pretty frequently between my Macbook and my PC.

My problem now is that the hard disk works perfectly fine for about a week, then it starts acting up. After I plug it in (it works in the few seconds after I plug it in) and leave it for a short while, it completely refuses to work, and just keeps giving me the spinning beach ball. Worse still, it hangs my Macbook and refuses to unmount. Even after I go ahead and unplug it, Finder will stop working despite my relaunching it and I am unable to restart the Macbook unless I hold the power button down.

After formatting the HDD, it works fine again. Unfortunately, after about a week or so the same problem occurs. I've been through this cycle three times now, and I'm getting pretty annoyed.

I really hope someone can help me out with this. I've tried looking online for solutions but I haven't been able to find any. Many thanks in advance.

 

The HDD is the original one that came with my Macbook, a Seagate... erm I'm not sure what other details there are that are relevant.

Oh, and another thing I forgot to mention - the HDD works fine on Windows computers even after it starts screwing up on my Macbook.

Also, the problem isn't limited to when I try and open files. It varies, but the HDD has hung both on attempting to open the drive in Finder, and working all the way until I try opening a certain folder or file.

Thus far the only suggestion I've found seems to be that I should change my casing (my current one is USB-only, firewire enclosures don't appear to be available anywhere where I live). Sigh...