Hi Bill and Aurick,
I have no problems with other applications saving to this folder or the hard drive. The drive is an external 250 gig HD but not a network drive. I have been saving projects to this drive for months from ZBrush without problems.
I typically create a client/project folder and then in that folder place subfolders each labelled with the application that is producing the enclosed files. So in the main folder is labeled JANNIS _PAIN/DEPRESSION. The sub folders are labeled : AE, EIAS,Poser, SAMPLES, Skeletal Anatomy,Tests, Ref.
Then in the Poser folder is a folder named ZBrush, and Z Mods.
The ZBrush folder was the original folder I was using for the ZBrush project files. When the error occured I created Z Mods in the event that ZBrush had a problem saving to a folder that it might mistake for its own application folder if there might be an issue with that. Z Mods didn’t work either. At that point I tried other things like recent projects that had saved properly in the event it was a geometry thing. That didn’t work so this morning I decided to try another HD. I successfully saved to the system drive. I then tried the 2 folders again and got the error again. In the event it was a hiccup of some sort yesterday. I then created a new folder named “crap” on the same drive on the main level ( named so that I would remember to dispose of it after the test). The project saved to this folder. I then tested the geometry that originally exposed the problem (A Poser model that my client wanted me to see if I could make it look acceptable for the project). That saved fine to this folder as well. So there you have it.
PS… I have saved to folders inside other folders before from ZBrush on this HD. The mystery continues… 
William Dempsey