External Hard Drive will not defragment

was trying to defragment my Seagate external usb hardrdive, when i hit the defrag button , it would start analysing as normal, then an error message would show up on screen, i think it said something about a missing file or corupted dll, i dont remember id have to check again if neccisary, but i cannot start the defragment process because it dosent finnish the auto analize.
ive tried restarting and tried defragging on a different pc. any other suggestons