What are you running the MDCs on? If Apple gear, did you enable Spotlight on the volume? If so, the mdutil processes hold on to the volume with a death grip, preventing eject of the volume. You can try to use launchctl to unload the mds processes but this works about 40% of the time. Otherwise, you need to reboot the machine and then eject the volume within the first ~3 minutes.
If you are on StorNext MDCs, then I am stumped as I have not seen this behavior.