Cannot Free USB Disk Space

I'd like to report a strange problem. I was backing up my desktop last night to an external usb disk last night. The backup failed to complete because I completely filled up the disk. df -h /dev/sdf1 1.4T 1.4T 0 100% /media/dz/5540d70d-... So this morning I deleted (using sudo rm -rf) an old directory on the usb disk which had about 30 GB of data in it. Now the problem is that df still reports the disk is completely full -- even after rebooting. /dev/sdf1 1.4T 1.4T 0 100% /media/dz/5540d70d-... Can someone please explain to what is going on here? Thanks David Zuccaro BSc (Chemistry, University Of Melbourne), GradDipCompSci (Monash), GradDipEd (Melbourne)