I realize that this is kind of an old problem, but it seems that no one has stumbled on the fix that we found…
After many searches and multiple websites, several calls to tech support (who were no help - they just said to return the drives), a colleague found an answer that worked every time for us.
We were setting up multiple MyBook World Edition II 1TB units and had this problem with upgrading the firmware on at least 3 of them where you get "The drive is currently busy. Please stop using all shares and try again." We did nothing but connect them to the network and turn them on. We could not set the units to Raid 1 for a Secure Volume, either.
In trying to affect the drive in some way (after much frustration), he browsed for the drive in the network (on XP), went to the Public folder and right-clicked to get the properties. Under the DFS tab, he cleared the History, Checked the Status and "Set Active". This last step seemed to do the trick, as the unit was not "set active" before.
After doing this step, the drive would do the firmware upgrade and allow the unit to be set for Raid 1 (a slow process, by the way).
None of the other fixes we've read seemed to work (resetting the IP, restarting, etc). We had no need to alter the units, as they are meant only as simple storage devices for Windows machines, but we certainly wanted them to work… Hope this helps someone else out there.