Defrag command line error - wrong volume type

I have 3 SATA drives; 2 in RAID0 configuration as the 'c' drive and a backup
device as the 'd' drive. I recently setup a scheduled task to defrag the 'c'
drive using the command line 'defrag c: -f' and all works fine.

Then I decided to do the same for the 'd' drive. the scheduler reports
errors and when I run 'defrag d: -f' from the command prompt it returns 'Disk
Defragmenter cannot run on this volume type'

If I use the XP GUI version of the defragmenter I have no problems on either
'c' or 'd' drives.

Simple question - why is the 'd' drive failing when using the command line
but not the GUI interface?

TIA