Problems renaming/editing SOME mp3/m4a files on external HDD

Please help!

I use Tag&Rename and MP3 Tag Tools to rename files / obtain track info / artwork (where missing)

My music is all on an external WD 750Gb hard drive (M:\music)

SOME files cannot be changed. I've tried using a different user account, updating the permissions/ownership, removing write protection... even using Godfather, alas to no avail.

Example: going through Bloc Party (M:\music\other\bloc party\)

Contents of "Little Thoughts Ep" renamed using %6. %2 mask, ie

01. Little Thoughts.mp3
02. Tulips.mp3
etc

Same thing with Silent Alarm...

tags exist, preview tab in Tag&Rename suggests everything should be hunky dory...
ERROR: cannot rename (filename) to (new filename).

At first I thought it was if the path was too long, but moving the folder to the root of the drive does not resolve...

Here's the "interesting bit":

Move the folder to a local drive (D:), rename files works ok.

Any ideas?

the songs play ok, I can rename using explorer, or by moving to a local drive and (tag&)rename, but that's a little bit tedious...

Any insights / suggestions welcome.

Many thanks

Jonny