Are you using a full drive defrag or just a file defrag? Are the scattered files normal ones or do they have names like $UsnJrnl? Sometimes these files are locked during normal operation and cannot be moved.
These are external drives connected via USB under XP SP2 Pro.
Size of the drive seems irrelevant, as it is common to have 40 GB free space on one drive, 200 GB free on another, etc.
Files are not system ($UsnJrnl/system volume, etc) but appear to be regular, normal files.
Drives are done with full defrag
Defrag is repeated a 2cd or 3rd time only to have the files still scattered.
A lot of files are consolidated, free spaces are much bigger & more contiguous, but it is bothersome to have free space for a few GB, files in a cluster, then another few GB free space.
I will try to check later on more of what the files are.