There is a difference between database fragmentation and disk fragmentation. It looks like this error involves the disk, not the database. And yes online DB maintenance will help with the database fragmentation, but also keep in mind that maintenance still does not remove white space. It never has. If large numbers of mailboxes get moved you will still need to do an eseutil defrag.
I think in your case a DISK defrag is in order, not a DATABASE defrag.