SBS03R2/Exch03SP2 I added up all users mailboxes and this equals 7gb. mdbdata is on e:\ partition for Exchange database and is 50gb /25gb free priv1 is 20gb. I can't see the reason for the difference really. I also noted the mailbox store has 4mb free after online defrag. Looking for some advice on how best to proceed, whether i must do an offline defrag of exchange db. thanks

Pretty sure this is due to items inDeleted Items Retention storage. These do not show in the usage stats. If you are tight on space you may want to look at winding back the retention period.