There is a chance that you haven't assigned a correct static IP to the nas. Some routers have their dhcp set up in such a way that only IPs within a range are acceptable for static usage. For example, my router works with IPs in the range 192.168.1.64 - 192.168.1.254. So, if I assign 192.168.1.12 to the nas then it is not going to work smoothly.
Besides that and in case the above is not your issue, I believe it is a windows 7 security problem and not a nas problem.
First of all, the EasySearch utility is not windows 7 compatible. Always run it with administrative rigths and in Windows XP compatibility mode (try it now).
Second, check if you have any weird jumbo frame settings in your ethernet card. This has caused me trouble before. If so, disable jumbo frames and test.
Third, check your windows 7 advanced network settings. Make sure network discovery is on etc.
Last but not least: I remember I had this situation once. As a solution I run a device-driver-like setup that is in the DNS-323 dvd. It installed something like a device driver... anyways, after that both the nas volumes were listed in my windows 7 and I could map them to drive letters again.