Well that was it!
Blame the way windows formats USB sticks as FAT!
I have just formatted the stick using the SD card formatter used to format the SD card now in the PA (as per the instructions)
It still days FAT on windows properties but it is now recognised by the PA unit.
And yes, when I plug it back into Windows, it comes up with:
"There is a problem with this drive. Scan drive now and fix it."
but still works.
....but I'll be using FTP from now on!
thanks for all the help