How to delete "invisible" files/folders (device backups)
The fact that the copy was instant would lead me to believe that the source drive and destination drive are the same. So I'm guessing that the Backup folder is actually somewhere on the MBP drive.

You could open terminal and do something like

sudo find ~ -name "3d0d7e5fb2ce288813306e4d4636395e047a3d28"

That would search your home folder for the backup file for the sms database. I just picked a random backup file that everyone should have :) as a way of looking for iTunes backups. I limited to the home folder try to speed up the search, but you could do

sudo find / -name "3d0d7e5fb2ce288813306e4d4636395e047a3d28"

and walk away and give it some time. Or keep subbing out different search locations like /Library, etc.

Future compatibility note: The different file permissions in macOS Mojave would likely prevent this from working. But in El Cap I think it should work.