HDD stops spinning every 9 seconds - help!

[amc]

I recently upgraded my iBook G4 hard drive to a 160G, and took out the old 30G drive and put it into a Transcend Storejet 2.5" external case.
It was working fine, but today when I plugged it in it started spinning as usual, then stopped after ~9 seconds. It started spinning again straight away, but every 9 seconds it stops and restarts.
It still seems to be working fine (in terms of writing/reading) but makes a lot of noise and I'm afraid that the stopping/starting isn't good for it. I tried unplugging it and tried it on a different computer as well with no luck.
Any ideas about what might be causing this or what might fix it would be awesome!

 

[deadpixels]

you're talking about the 30gig, right? when hard drives start making alot of unusual noises, it's soon the end! you should run a hardware SMART test on it

 

[amc]

Yep, its the 30gig I was talking about. Strange though - I was having the spinning/stopping problem when using the HDD with old G3 (yes OLD!!) power pc's at my university, but at home on my iBook it's spinning fine...
Could this be a problem with the power supply through the USB being insufficient? Or being connected to the older computer? I tried to check the SMART status but it says that it's "not supported". Is there another way for me to check if the HDD is okay?
Thanks for the help!

 

[deadpixels]

so, is the problem only when the hd is inside the external case?? then it might be compatibilty problem between the case and the old hd? although i can't understand why