Time machine preparing backup for hours
I have been backing up my Time Machine since 2011 on my Macbook Pro Mountain Lion with no issues. I don't have a lot on my computer so backups were always a breeze. This week I upgraded to El Capitan and wanted to back it up, but for 4 hours it seemed to be stuck in "preparing backup" mode. What could be the problem? Thanks.
MacBook Pro, OS X El Capitan (10.11.3)