Time Machine won't backup a folder that was previously excluded I was troubleshooting my wifes iPhoto library and rebuilt a duplicate library.  I didn't want to fill up the 2TB external hard drive used for Time Machine, so I excluded the folder that contained only the rebuilt library.  I now have deleted the old Library and want Time Machine to backup the new, rebuilt library, but it won't.  I moved the rebuilt library to the ~/Pictures/ folder and deleted the exclusion from Time Machine, but the Library doesn't get backed up.  I deleted the com.apple.timemachine.plist, formatted the external hard drive, and started all over with a new Time Machine backup, but the iPhoto library still won't get backed up by.  

What's weird is that when the first Time Machine backup begins, it says the backup size will be ~1.1TB (which is correct), but once it's complete, the backup is only ~600GB (iPhoto library is ~500GB) I know Time Machine won't backup iPhoto if iPhoto is open.  iPhoto is closed the entire time.
I had the same issue. The workaround was that adding the folder to exclusion once and then start the backup. When backup is done, remove it from exclusion list. I am still using Mac OS 10.9.5. So, I don't know if this bug has been fixed on 10.10???