Disk space issues

I am really struggling to understand disk space on my Rockstor installation. I have a pool that is built from a 5TB disk and a 2TB disk. These disks are built on hardware RAID1+0 and RAID1 volumes, so I set them up as single volumes in rockstor. Until recently, I had 1.6TB free on the pool and share. When I run du against the mounted volume, I see that 5.3TB is used. This all made sense to me.
I recently setup the Duplicati rock-on and attempted a backup of about 1.6TB of the data to back blaze. I had initially configured Duplicati wrong and had the remote volume size set to 2TB, believing that would be the max uploaded data, when in fact that is the remote block size for data. I believe that Duplicati cached data for the upload and ran the volume out of space, but I don’t see anywhere on the system where the data is hiding. The Duplicati-config volume has 12GB of data in it. The Duplicati rock-on has access to my 7TB share, but I can’t seem to find any large files that are causing the loss of space. Btrfs shows that the there is only 1.95GB of unallocated space. A balance did not restore the missing space either. I’m at a loss to determine how Duplicati ran the volume out of space, but there have been no other real additions to the share except a few GB of photos. Any idea where to look?

Here are some outputs from the Btrfs fi tool:

[root@rockstor mnt2]# btrfs fi show /dev/sdb
Label: ‘Pool0’ uuid: 6b837d78-38b7-468e-8c14-9a458f6c9bfe
Total devices 2 FS bytes used 6.89TiB
devid 1 size 5.00TiB used 5.00TiB path /dev/sdb
devid 2 size 2.00TiB used 2.00TiB path /dev/sdc

[root@rockstor duplicati-config]# btrfs fi usage /mnt2/Pool0
Device size: 7.00TiB
Device allocated: 7.00TiB
Device unallocated: 1.95GiB
Device missing: 0.00B
Used: 6.90TiB
Free (estimated): 12.51GiB (min: 11.54GiB)
Data ratio: 1.00
Metadata ratio: 2.00
Global reserve: 512.00MiB (used: 0.00B)

Data,single: Size:6.89TiB, Used:6.88TiB
/dev/sdb 4.94TiB
/dev/sdc 1.94TiB

Metadata,RAID1: Size:25.14GiB, Used:273.12MiB
/dev/sdb 25.14GiB
/dev/sdc 25.14GiB

Metadata,DUP: Size:31.46GiB, Used:11.10GiB
/dev/sdb 31.95GiB
/dev/sdc 30.97GiB

System,RAID1: Size:32.00MiB, Used:80.00KiB
/dev/sdb 32.00MiB
/dev/sdc 32.00MiB

System,DUP: Size:32.00MiB, Used:720.00KiB
/dev/sdb 64.00MiB

/dev/sdb 999.00MiB
/dev/sdc 993.00MiB