I've tried both GDS and MDS and neither could index a volume mounted on a folder. I couldn't find a way to force MDS at all. I did manage to add the folder in GDS but it didn't really index anything even when it shows "up-to-date" on the status. Strange enough, it does have 2 files indexed that I can confirm. I wish there's a way I can "browse" what's indexed.
The other thing is, GDS couldn't index my Edge DiskGo USB drive. I have the locking turned on but the time I tried to add the drive to indexing, it's unlocked. GDS just said "invalid location".