Transferring files to networked drive

I've got two ATV1's connected (wired) to my airport extreme, both mounted to the 2TB external HD connected to my airport extreme (USB connection).  My goal is to add a 3rd ATV (black) and a network connected external HD....

....BUT....

Once either ATV is showing the drive as mounted, I can no longer transfer files to my external HD via my mac.  I get the following error:

The operation can't be completed because an unexpected error occurred (error code -50)

Any thoughts??  I'd love to be able to figure this out.  

FYI...

My external HD is formatted for the Mac (not NTFS or FAT32).  Also my Max is running OSX 10.6.6.