You said, “on the old drive, after moving the mailboxes it didn’t increase my size by 8Gb?”. I think you must mean it did not DECREASE the mail store on the old drive. That is because when you move mailboxes it leaves behind white space. You will need to do an eseutil offline defrag on the stores you moved the mailboxes out of to reclaim that space. Keep in mind the space requirements to do the offline defrag. You do not have enough space on the old drive, however you can use the new drive for the defrag temp file.