zimbra NE 5.0.15 strange full backup behaviour

[djingo]

Dear all,
we had some problems trying to restore in time one users account from the backups as the restore always restored to the time of the restore and not to specified time. (user was looking for some deleted emails)
So I wanted to make sure that all backups were running fine and moved all /opt/zimbra/backups to a renamed folder to launch a completely new fresh full backup.

The strange issue is that the first full backup took about 19 hours to complete while checking the previous full backups (with zmbackupquery) they took about 3 hours. Is this behaviour normal for the first backup done in the "clean backup folder" ? (maybe due to the way ZM5 makes full backups with the hard links?)

The disk on which we are backing up is an external disk due to disk space on the internal raid.

Thank you for your help,
Mark