ExtPart threw this error on a Windows Server 2003 R2 64bit system that just had logical disk 0 extended.
ExtPart does support 32 and 64bit Windows Server 2003 R1/R2 (needs to be extracted via 32bit Windows or a tool such as 7zip).
My guess was that the command interpreter was not "seeing" the newly extended local disk space. Sure enough, a reboot fix 'er right up!
I highly recommend both tools be a part of any administrator's toolbag:
Dell's ExtPart Tool