When you use the SBS move folder wizard, you receive:
Insufficient space on target volume.
even though you have more than enough freespace?
The freespace variable is a DWORD that can store a maximum of 4294967296.
If you had 4294967297 bytes available, the variable would overflow, with a resultant freespace of 1 byte. This happens again at ~8GB, ~12GB, ~16GB, etc...
The only work around is to create a temporary folder and copy enough files into it to reduce the freespace below one of these ~4GB increments.
After the move folder wizard completes, you can delete the temporary folder.