Hello,
maybe it works, if you unplug the external disk before creating that shared folder within the console.
The other, not supported and not recommended option would be to login to the server directly or via remote desktop, create a folder there, share it manually and adjust the share and ntfs permissions manually. Since this share is not under control of the WHS console, this is not the best solution, especially if it is a drive included in the storage pool.
Best greetings from Germany
Olaf