Well you might try mapping the drive to a different drive letter.
Do you have the same problem if the drive is mapped to a different letter?
My main system reconnects just fine with both my DNS-321 NAS devices ( drive 'X' and 'Z' ).
Main system is Vista 64. My secondary system is running MCE2005 and no issues with losing drive mapping there as well. My wife's laptop is running Windows XP PRO and no issues with drive mapping there as well. Oh. my other laptop is running Windows Vista Home and it also has no issues with losing drive mappings.
But you said you have Windows 7 64BIT edition.
Well I just booted Windows 7 64BIT and I mapped both NAS devices. Next I rebooted.....
Never prompted for password and both drives are green. Access to the drives is fine as well.
I am not using any group settings and I am running with the latest beta firmware.
I have two 1.5TB Seagate drives on one of the NAS units and two 1TB WD Green drives in the other NAS.
If you right click on the drive and check Properties/Security what do you see?
Mine shows "Everyone, Root(Unix User\Root), Root(Unix Group\root)"