Zone Image blank when setting up zones

[cat007]

Hi all.
I'm using Zoneminder on a R-pi 3 with a RTSP stream from an IP camera.
Everything seems to work as expected but I'm having issues when trying to set up a zone for motion detection.
The 'Zone Image' is just blank.

I've tried Chrome and Firefox - still no dice.

Any ideas what might be causing this?

Cheers
Hunt

[cat007]

A small update.
If I set the images path back to default, i.e. just put images in the box, it works.
If I set the images path to the external hdd I'm using, i.e. /mnt/usb/zoneminder/images it doesn't work.

Any ideas?