Frigate - probably more of a Linux question, though

I’m running Frigate on a separate Pi4 and have a 1TB external USB HDD connected to it.
The HDD is auto-mounted to /home/pi/frigate/storage_usb and everything is saving fine to the ‘clips’ and the ‘recordings’ folders - got a Samba container running on the Pi and I can access everything fine from my Windows PC.

I’m running into an issue, though, when the total size of the /storage_usb folder exceeds the size of the mSD card (128GB in the Pi4.

Looking at the Pi with df -h I see this:

pi@fripi:~ $ df -h
Filesystem      Size  Used Avail Use% Mounted on
/dev/root       117G   74G   38G  67% /

As soon as the ‘Used’ number hits 117G the Pi starts acting up, i.e. Frigate does not record any more and I lose access to the Pi, although the USB Drive is still 90% empty.

Anybody got any ideas on how to fix this?