How to : MAC OS client can write to hard disk (NAS) device mounted on router with professional Firmware.

To have write access from MAC, re-check file sharing settings.

In System Preferences , click "Sharing"  and turn on "Windows Sharing" for 10.4 or in 10.5 “Share files and folders using SMB.”
"File Sharing" in 10.6

Modify User privileges to have read/write access to shard folders.
The default privilege for a user account or group you add is "Read Only".

To modify user privileges, click the menu next to user and select read & write option.