A:Answer To comment on Belkin support answer:
The problem seems not to be with the partitions, but rather with the router not recognizing GPT (http://en.wikipedia.org/wiki/GUID_Partition_Table) and can only work with MBR (which is 2TB limited).
If that's true, then regardless of how you partition, you will not be able to go beyond 2TB which is MBR limitation.
The solutions is rather simple: release new firmware which has the Linux kernel recompiled with GPT support.
Otherwise you must state this limitation upfront and reimburse the customers who spent their money buying this router and useless hard disks.