upgrader

I seem to be having a problem with upgrading a database to 8.82 PL5.

When going through the upgrade wizard, I get stuck at the part where it asks me where to store the backups of the database, the database I'm trying to upgrade is ~25 gb of space. I select my D drive which has around ~29 gb of space, so there should be at least ~4 gb of space left after the upgrade. But when I get to that screen it only says I have ~4 gb of space available. It seems to have already counted the database within the available space. I've never seen this behavior before. Any suggestions?