DF v2.03 reports wrong amount of free space

WinXP SP3

Defraggler v2.03

external 1.5 TB USB-drive w/ 1 NTFS partition

Drive/Partition as per Windows drive management:

capacity: 1397,26 GB; partition: 1171,87 GB; not assigned: 225,38 GB

Drive parameter as per Defraggler:

capacity 1171,9 GB; used: 913,3 (78%); free 258,6 (22%)

So far, so good ...

After starting the defragmentation Defraggler reports:

Less than 10% free, ... will take long time ...

This message seems to be wrong (regarding the free space ;))

Regards from a happy user.