I've been running a 5-drive software RAID under Windows 2003 Server. When it fails, it causes really strange problems that are difficult to deduce. After the third occurrence, I've decided to try using a Drobo instead, connected via USB2.
The Drobo drive is mapped as Z:, and has a single folder in the root called Repository, which houses all other folders and files. I've shared this folder, and added the necessary permissions. I can map a network drive letter to the Repository Share on the server and others on the network. However, it always appears to be empty (even on the Server).
Just to verify that I've used the correct approach, I've performed these same steps using a folder on the Server's C: drive, and it works correctly.
I've found a number of references to people having troubles with sharing folders on USB drives, but I didn't find any definitive solutions. (One individual suggested mapping the USB drive to a local NTFS folder, rather than as a drive letter, but the option was disabled in the drive management settings.)