I have the same problem, a 750GB USB drive attached but receive an "access denied" any time I try to write to the drive or change permissions. Here is the setup information:
Win2K - Logged in as local admin
LinkStation (LS) HD-HG250LAN Firmware V1.34 (US)
Seagate Free Agent 750GB drive, formatted as EXT3 via LS GUI
Read the owners manual front to back, searched the Knowledge Base and Forums
Rebooted Link Station after format of USB drive, PC several times, logged in to LS via web GUI as root, permission change attempted via Windows Explorer as both local and root
"USB drive as shared folder" allowed in LS menu USB Setup
USB drive recognized by LS (usbdisk1) and visible to PC via mapped share and Network Map, but read only allowed
Backup of LS shared folder via LS GUI to USB disk failed
Any suggestion you may provide will be appreciated.