Hi the disk might not have the required space to perform the whole backup opperation.
After reading your post, my first advice goes to tell you that for a (real) BACKUP,<b> you need</b> to have more than one copy of what you’re backing up.
When you say that you prefer “not to wipe the drive” I assume that it might be for not having any other backup.
If this is the case, rethink it.
After you have more than one backup of what you’re working on, wipe the drive and strat from scratch.
What’s the size o the drive in question?
What’s the size o the data you need to backup?
What is the data retention policy for that data?
If you need further assistance, please post back.