As found on the Western Digital forums here, it sometimes occurs that an HFS+ formatted USB results in an error "share cannot be found" when it is mounted via the Finder, SMB, of AFP. The error states "The operation can’t be completed because the original item for “usb1-1share1” can’t be found."
The problem is caused by a corrupted .AppleDB file. The fix is simple and must be repeated for each partition and share on the USB disk:
SSH into your device.
cd /shares/usb1-1share1 (or whatever your USB share(s) are named)
mv .AppleDB broken_AppleDB
The share should now be accessible.