Second hard drive hangs

Hi,
Just discovered that my second hard drive has started to unexplicably hang and crash any program using it including finder.
It's a work computer and its the hard drive that contains all the work files since the copmuter was put in.
It was working fine, and then I went off for 2 weeks holiday and now it hangs. Does it in Finder which I can deal with, but it crashes Pro Tools whenever I try and navigate to the hard drive. I've created a temporary folder for the work on the Mac HD, but I need to access the files on the other one... Any ideas?
Running a 'Dual 2Ghz PowerPC G5'
Thanks.

 

There are some backups, but they're not very recent..
I don't think the disk has died, as i left the 'open' dialogue in Pro Tools up for about 10 minutes or so and one or two folders had come up (program still hanging though), I tried this in Finder, but there was nothing, had to force reset the computer by holding the power button to 'unstick' it.
The computer and drive were purchased in mid-late 2005.

Tried doing disk repair but got an error:
'Could not unmount disk'

What does that mean/how do i fix it?