I have tried 3 different 2860's now & yes some USB memory sticks appear not to work like my Corsair Voyager but it says no disk detected when this happens....
I have added a 1GB drive & it shows in file explorer & I can manually copy files to and from it but the logging creates SysLog folders but no actual log files.

Most frustrating - firmware is 3.7.3.3

Any ideas

There is a newer beta if you want to try at one of your 3 units, which fixes many issues
http://www.forum.draytek.co.uk/viewtopic.php?f=19&t=19313&start=80#p80330

To enable or use usb syslog you need go at USB Application>submenus
and to see the logs: Diagnostics >> Syslog Explorer (top next tab of web syslog)

Is there a list of the "many fixes" anywhere?

The admin may know (later when comes) what are the fixes ... usually with beta are not any released notes

OK I will await their input...

I was actually only on 3.7.3.3 so have just upgraded to 3.7.1.1 first...

oops i upgraded to 3.7.4.1 Same problem though.... USB disk works & creates folders but no SysLog data...