Can't access USB thumbdrives

Asus Zenbook
Debian 13 Trixie
Mate DE

I have just noticed that this computer has stopped recognising USB
thumbdrives. 

They are listed in lsusb, but do not appear in my file browser (caja),
nor in df. 

The same machine sees the same USB thumbdrives when booted into
windows, so it appears not to be faulty hardware. 

I can't say when this started happening; I haven't plugged in any USB
thumbdrives for a few weeks. 

Despite all this the machine boots from an external USB-connected SSD.
It's using wired ethernet via a Startech multi function USB hub, so
it's obvious that the USB is largely working. I have tried connecting
the thumbdrives directly into the computer's USB ports (i.e. not via
external USB hubs) and no change. 

Any suggestions? Any other info required?

Chris