As far as I know, this requires "USB Host" mode which the A100 lacks (but the A500 possesses).
And also AFAIK, the ICS (Android 4.0) update and recent firmware updates did not change this fact (so it is appears likely that the lack of functionality is hardware based).
It is a shame, as sites like Amazon are vague about the use of USB, mixing terminology for copying files to the Acer itself vs. playing files off an external drive. But I think it simply does not work on the A100, even if the external HDD is fully powered on its own.
I'd love to be proven wrong, so will be subscribing to this thread! *smile*
Thanks,
sutekh137