pCP 8.1.0 doesn't detect USB hard drive.

This is a bit strange to me...

Raspberry Pi is Model 3B. I am using a fresh installation of pCP8.1.0. I have expanded the filesystem to fill the 16Gb SD card. I've installed LMS, a few extensions such as nano, filesustems, util-linux.
When I boot with a 2Tb Seagate USB hard drive (Ext4 format) attached, pCP doesn't seem to detect it (the "Pick from the following detected USB disks to mount" pane on the LMS tab doesn't offer it as a detected USB drive).
I notice that if I log in to the Pi via ssh, I can mount the drive manually via the commandline, and LMS can access the music files.
This Raspberry Pi used to recognise the very same drive before (I think I was using pCP 6.1.0 then).
I have tried two USB cables.
I have increased waitusb to 6 seconds - doesn't make any difference. Should I increase it further?

I have two Raspberry Pi devices (3B and Zero2) with SSD drives connected via USB, and this same Pi used to work with the same USB hard drive.

Any advice?

Robert