What is the drive size, Mfr and Model of your external drive your using?
Yes, I saw this same message and was unable to mount the drive at all. I was able to mount it using other router models I tested. I used OSX 10.7.5 and 10.8.3.
Can you tell me if you saw a "No Name Network Drive" in the SP app? I saw this as well which was different from the other routers I tested. The other routers showed an actual name of the drive. I was able to mount the drive on a Windows 7 PC.
Please phone this in to DLink support, ask for level 3 or higher support as this can't be resolved at the lower levels. I have sent this into my contact however I would like for you to phone this in as well. This will help get more visibility with DLink and hope it will get fixed sooner.