I have a similar issue...if I reboot computer because of an update, the DNS-323 as drive X goes away and I am unable to go back into Easy Search Utility and assign a drive letter to the DNS...I am stuck in the loop of 1) highlight volume, 2) select drive letter X, 3) Click connect, 4) Login at Login popup window, 5) Error window popup with message "Failed to Map Drive". At this point the DNS-323 is useless to me. I have been dealing with this and similar issues with the DNS-323 for two weeks...I have yet to get the DNS-323 to do what it is supposed to do. This is set up as a RAID1 drive with two 500GB HD's. How do I get out of this "Failed to Map Drive" loop? 
I had this problem. I found that the operating system had already assigned the drive letter even though it wasn't showing. You can't disconnect the drive and you can't assign the drive letter in the easy find utility. The way I fixed it was to use the command prompt and manually disconnect the drive them re-connect with the easy find utility. The command prompt commands are as follows:
To disconnect a mapped drive: 1. Click Start, and then click Run.
2. In the Open box, type cmd.
3. Type net use x: /delete, where x: is the drive letter of the shared resource.