Currently, if a user includes sub-brick selection (or range selection, I imagine) in the source argument to 3dcopy, they get the general "** FATAL ERROR: Illegal old dataset name! - EXIT".
Checking specifically for use of either type of selection and giving a more specific error (simply saying it's not allowed) could save users time trying to find a problem in their filename and perhaps spare you guys a message board question along the lines of "why didn't this work?" when they don't find one. :-p
Also, you might consider pointing out in its help doc that 3dcopy is intended only for whole bricks, and not, e.g., extracting a brick from a bucket.
Thanks,
Terry