please recommend a disk-exercising program?

[John .]

Hello list,

Can anyone please recommend to me a program that will stress-test a
disk? If it matters, two are connected by firewire, another via usb.
Filesystem is ufs. I think I have a disk that might be about to fail,
but it only sometimes errors when under stress. I need to see if it
was a fluke or to really make it fail, so that I can get rid of it.

thanks
-- 
John