Help! Cannot recover space from deleted files

Found the solution in this post:

https://community.netgear.com/t5/Using-your-ReadyNAS-in-Business/Deleting-Files-and-Snapshots-does-not-free-up-space-UI-shows-no/td-p/1103450

Basically, my ReadyNAS had snapshots that did not appear in the GUI, so I could not delete them.

I used:

btrfs subvolume list -s /data

to find the snapshots, and then:

btrfs subvolume delete <full path to volume>

to delete them, recovering almost 2TB of space!