Sorry
as long you are aware it use only FAT32, take a look at the user guide section 3.18 (page 386) if you have set something wrong
ftp://ftp.draytek.pl/Seria_2860/Instrukcje/V2860_UserGuide_v2.5.pdf
Try contact the support if someone else will not reply
It may sound silly suggestion but i have seen a case (not with draytek) so swap the usb port just in case there is a bug at the one you are using now

Ooops.... I feel immensely stooooopid now...

Just tried a different #USB key and realised that the old one was formatted to FAT not FAT32.
I reformatted as FAT32 and now files can be written to it

Big der to me..... :mrgreen: :mrgreen: :mrgreen: