For those interested this MAY work. I know it worked for my Drobo Pro after much trial and error
So I plugged my drive, 12TB Drobo Pro into the USB port after flashing ATV and it would not pick it up at all. I tried all possible formats. FAT32, HFS+ and NTFS. (In the case where its stated that ATV does not recognise NTFS in this case was just not true. It eventually did. As for FAT32 it was WAY too big a drive)............No luck at all. I figured the drive was just too big and ATV was going WTF??? Are You kidding????. So I played around some more and created a second partition (volume) on my drive.....in essence duplicating it in its full size (Remember in doing this, make sure the drive is blank). So In My Computer I went from having Drive E: 12 TB NTFS to Drive E: AND F: NTFS 12 TB. When I plugged the drive then back into the ATV and went to change from internal to external it picked up "both drives"..........Keeping in mind there is only one physical drive. Now am not going to even pretend I know how this happens, all I'm saying is it did and does. So I have 2 drives now listed E & F. When I go to Maintenance and wish to switch to external (though ATV does not actually state which is which).....I pick the lower on the screen and ATV goes ahead and formats it to HFS+ "MediaHD". I plug it back into My PC and I now have E: still NTFS, and F: HFS+ format drives. I'm using MacDrive so My PC can read Apple format drives.........I also have a USB switch so I can change from connecting my external drive to ATV or My PC at the push of a button. This is so much better for me for copying media to my storage. (Bearing in mind that this is a wired setup)
On the F: Apple drive now is among other things a folder called External Drive which I drop all my i.e. DVD files into and when connected back to ATV are played under NitoTV "Files"
[b]Creating a new volume:[/b]
So basically if your at a complete loss.......try getting a blank external drive, create a second volume on it and see if it works. I'm not swearing it will work, I just know it did for Me.
One other point.......My experience up to now.........The files that are written onto the external HD during the ATV format - folders and files etc. after you have switched to external, should be left alone........meaning.......unless you know what your doing and know how to path out your media folders from XMBC, Boxee etc , I suggest not renaming your folders to "My Music or My Movies" or the like. Its not fatal, it can just get messy.
Hope this helps some people out.