Folder Incorrectly Identified as Full

[pmfortune]

I have a number of folders storing music on my NAS server all sitting on a single system volume.

There is over a terabyte of space left on the volume but if I try and create a new sub-folder in any of the top level folders I have started getting the message "The operation can't be completed because the disc "**** Music" is full. Remove files and try again"

I've rebooted the NAS to no benefit.

Any ideas how to resolve this?

(TS-435A running V 5.0.0.1858)