Hi everybody. I think I've found the issue, and found a solution. The problem seems to occur specifically with the Seagate Barracude 7200.11 drives with CC1J-firmware on it.
When different harddisk-firmwares are mixed in a Raid-5 array, the disks with the CC1J-firmware are often not recognized on warm reboot. I now have a set of 4 drives all running CC1H-firmware, and the unit seems to be running properly. But as soon as I swap one of the CC1H-disks with a CC1J, problems occur on warm reboot.
So, if you have a set with mixed firmwares on the drive, always do a full shutdown, and reboot "cold". If you have a matching CC1H-set of drives, as far as I've found, all is fine.
Seagate were very helpful and responsive in my email-correspondence with their tech-support staff, and they provided me with the option to flash the CC1H-firmware to the drives I purchased. (Which were shipped with CC1G-firmware originally.)
One note though: I tested the same drive with CC1J-frimware on a DNS-323, and, even formatted as a stand-alone drive/volume, the drive has problems being recognized on warm reboots. (running 1.06 firmware on the DNS-323, btw.)
So, as far as I can see, and based on what I have tested CC1J-drives are problematic in the DNS-units, CC1H-drives seem fine.
Hope this helps any other user with the same/similar problems.
D-Link staff - any input/insights/comments from your end? Thanks!