I think most replies have the right idea ....
A new power unit is relatively easy to change ... but do be careful ... they contain BIG capacitors which can store a big charge for weeks!!!
Not expensive is good ... but cheap isn't necessarily the way ... a cheap powerunit is like building your house on sand ... and nov****h have a reputation for burning out after 1 month 1 day go for a mid-range one instead
Does your local tip have a reclamation centre?
do you pass any corporate skips?
both are good sources of older (but still good) spares
failing that a USB enclosure costs much less
http://froogle.google.co.uk/froogle?q=usb+hdd+ enclosure&scoring=p
and is as easy to fit