Timeshift issue or have I mounted a drive incorrectly?

I’m trying to setup Timeshift (via rsync) on my new Kubuntu system, and Timeshift is reporting “Not enough disk space (<17.6 TB)”. I have the OS installed on the 1TB SSD, and want the backups to go to the 4TB HDD.

  • 1TB Samsung SSD (921GB free, sda2, Ext4, mounted at Filesystem Root)
  • 4TB Seagate HDD (3.9TB free, sdb1, Ext4, mounted at /mnt)
  • Kubuntu 18.04
  • Timeshift v19.01

Could this be a Timeshift issue, or have I possibly mounted my second drive (the hdd) incorrectly? I also am connected to a couple of old servers here at the office, but not anywhere near 17+ TB of storage between them.

Please note that this is the first time I’ve attempted mounting a new drive in a system under Linux, so it’s very possible I’ve done something wrong here.