Can't Write to FLAC files (Vortexbox)

I am in the process of ripping my CD collection to a Vortexbox appliance (FLAC files). The VBA is working fine in terms of ripping and playback (through SB Touch). But I am having trouble using mp3tag to tag files. Here's what happens:

When I edit the tags on the Vortexbox appliance files and then hit Save Tag, I get a message saying " cannot be opened for writing". This happens for every attempt to retag no matter the ripped disk. (Yet the ripped disks play fine.) If I try to import outside tagging (MusicBrainz) the same message appears when I try to save the imported edits.

When, however, I use mp3tag to edit the music tags on my C:/ drive (which are Windows Media Audio) files, I have no problem. It accepts the newly edited tags.

Any ideas what's going on? I know others use mp3tag for vortexbox.

I am running Windows 7/64.