UD checks for a disk busy error when a disk is unmounted and if it is busy, it checks for open files. If no files are open, the unmount is forced. If you look at the log, you'll see the log messages showing this action. If UD operated as you suggest, and this situation was to come up during a shutdown, you would get an unclean shutdown and I'd be hearing about a 'bug' in UD that would not allow the array to shutdown cleanly.
The problem is the mountpoint could not be removed because you were on the cli accessing files at the mountpoint.
While this situation might be an annoyance to you, it is really the best way for UD to operate. Fix: Don't unmount a disk if you are in the cli on that disk.