Thanks again for your replies mlord, much appreciated. The drive that contains the subdir im trying to write to is a 2TB. It has ~5GB left, and my test file is say 9GB. When I cp this file to my mhddfs mount, instead of writing the directory structure to another free drive and placing the file there it just fails with 'out of space' error.

If it helps my move size limit is at the default of 4gb