Are these multiple drives of the same brand and model that is connected simultaneously?
BTW...which hub(s) and drives are you using?
Same brand, same file system, different data.
x2 Western Digital My Passport Essential WDME3200TN 320GB USB 2.0 Black External Hard Drive
and a DUB-H4 (http://www.dlink.com/products/?sec=1&pid=152)
note, the hub is powered via external power adapter.