Windows Boxes Losing Sight Of Drobo S via Airport Extreme

Hey all

I have my Drobo S USB connected to my Airport Extreme and the disk is shared via login/password and is formatted HFS+.

On initial startup of everything - router, airport, Drobo etc my Drobo can be seen by all my Macs and Windows (XP, Vista,7) machines. All can happily read and write to the Drobo.

After a few hours though ALL Windows machines loose sight of the Drobo and the network drive dissapears from network places. Attempting direct access to the Drobo on the Windows boxes just times out. My Drobo is still visible however with all my Macs.

Anyone, have any ideas what might be causing this, it is painful to disconnect all users from Drobo and re-boot. I have a feeling it may be Time Machine causing the problem. Time Machine backs up to the Airport internal disk, not to Drobo.