I thought maybe Nexus Media Importer would do it, I couldn't figure out how to copy entire directories from my /SD Card folder to the USB drive though :/
ES File Explorer (under KK) doesn't show the USB Drive like it used to under JB (Storage USB 1.1.1) which is a shame so I too wouldn't mind knowing a file explorer app that works (and allows write to the usb drive without rooting)
Kinda pissed Google broke USB OTG for KK, have you ever seen how slow Airdroid is at transferring big files over wi-fi...
It's not ART related, google are just gradually locking down their phones more and more which is to be honest, starting to get annoying. It's ridiculous to need root just to be able for example: to turn on/off GPS or Airplane mode. It's getting stupid now. Google are you listening?