I have experienced the same issue.
Environment:
Windows 7 SP1, IE9 & Firefox 4DNS-321 Configuration:- Firmware: 1.03
RAID: RAID1 with 2 x 2TB Seagate Barracuda 5900rpm, 32MB, SATA 3G hard drives
Services: iTunes service on the unit is enabled.
Networking: Netgear WNR834B b/g/n, 10/100 Router
NOTES:
I created two shares on the DNS-321, one for primary iTunes storage and a 2nd for system backups. I was able to transfer my iTunes library of 40GB to unit without issue. I never got to setup system backups.
ISSUE:
After about a week, the DNS-321 stopped responding. I could not connect via mapped drives nor via web server. Using the easy finder on the included CD, I was unable to see the unit on the network. The lights on the front of the unit were all still on. I tried to power down the unit but the power button was not responding either. Finally I had to pull the power cable on the unit to reset it. The unit came back online for about 10 minutes then I lost connection again.
After reading the forum and resetting the unit by pulling the power cable again, I tried reapplying firmware 1.03. After the firmware applied the unit went offline again after its reboot within 10 minutes. After another power cable reset, I went into the web server and turned off the drive hibernation. No improvement. I downloaded all of the posted firmware updates from D-Link's FTP wondering if I rolled back the current update or even reapplied the firmware updates in order, it might resolve the problem. Unfortunately, the DNS-321 will not accept firmware updates older than the one currently installed on the unit. So reapplying firmware 1.03 is possible but not 1.02 or earlier.
To rule out my router, I direct connected to the DNS-321 with a crossover cable, but after 10 minutes, I still lost connection.
Finally I used the reset button on the back of the unit. The unit came up and after 10 minutes I still lost connection.
The exception to the 10 minute rule has been if I get on the web server and go to look at the iTunes configuration. The progress bar comes up, gets to 3% and then I lose communication with the DNS-321 immediately. It was this issue that made me think I might have better luck fixing the issue if I rolled back to an earlier firmware update which specifically stated it fixed the unit's iTunes config screen lockup.
I have concluded this to be a software issue. I am not sure if the DNS-321 still has issues with larges hard drives or if there is some other issue, but I am eager for a resolution. Even if there is a beta fix out there I would be willing to give it a run.