Mirroring won't delete old files resulting in Back up disk getting full MAC

[guywithmac]

Hello Everyone,

I am an happy user of FFS since years with my Windows based laptop and never had a problem.
Recently i bought for work a MACbook and i have a big problem with it and FFS.
I am mirroring my entire laptop onto an ExFAT partition of an external HDD (WD My Passport Ultra 1 TB). I choose ExFAT because it allows me to read the drive with Windows based computers as well as macs.
The problem is: everytime i try to Sync my back-up, the new files are added but the old/deleted/moved files won't be deleted so my back up got bigger than my original Volume till now that is completely full and won't sync anymore.
I am using mirroring mode and the "delete files" in settings is ticked as Permanent.
I am running OSX 10.9.2 and FFS 6.9 (64 bit).

What am i missing?
Any Help will be very much appreciated!
Thank you very much!