Sometimes, the external hard drive enclosure's power supply can be bad.
Or there may be a switching circuit inside the housing that has gone bad.
There are several items which could be acting up; and between the HDD
brand manufacturer and the external enclosure manufacturer, there should
be some glimmer of customer support from their home web site links.
Usually, when a certain brand hard disk drive acts up, in the past that meant
one would try a different brand. And if an enclosure acts up, it could be just a
power supply going weak (or failing, since some supply 12v/5v/2v, etc at the
same time, via several wires, inside the housing) and it may be time to get a
different brand enclosure, too. Some kits include differing brands of HDDs.
Some cited examples of reasonably good new hardware include drives from
OWC, such as those which support booting FW400/800 and USB2.0 with Mac
and have an external power supply; good for new and older Macs, & PCs.
The product you have appears to be fairly new, within the past year:
http://www.macally.com/en/Product/ArticleShow.asp?ArticleID=337
However, there may be other things to consider; between the major components
and the important minor players within the device. Power supply? Maybe.
Good luck & happy computing! 🙂