Qubes 3.2 backup of template VM fails - device out of space

[cybe...@national.shitposting.agency]

I have a template VM im trying to back up which is rather larger, 185gb or so.

Am attempting to backup to an external usb-connected sata drive with about half a terabyte of space on it.

Less than 10% into the backup a failure message appears informing me that "device is out of space" where in reality the device has only had a small portion of it's 500GiB used.

actual file size of backup after failure is only around 100MiB