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.