SMB Disk share does not work on airport extreme (Simultaneous Dual-Band II)

I've just got a brand new airport extreme (Simultaneous Dual-Band II). I bought it for the fast N network and for the NAS functionality provided by the device.

I have connected a HFS+ formatted USB disk and it works flawlessly with Macs on the network using AFP file sharing. However, SMB/CIFS filesharing is really poor. It work for about 10 minutes after a reboot of the airport express. After this it is no longer possible to mount the disk over SMB - neither from win7 boxes or from Macs using the smb:// mount option. Any SMB connected drives will also disconnect from the Macs at this point. AFP connected drives still functions without any problems.

This is extremely annoying since I would very much like to access the drive using the SMB protocol, not only from windows machines but also from other devices (which naturally don't use AFP).

Does anybody else have SMB working on the new late 2009 airport extreme (Simultaneous Dual-Band II)? The unit runs firmware version 7.5 and there don't seem to be a downgrade path for this particular device.

Any ideas on getting this to work? Otherwise it is mostly useless as a general NAS solution...