USB drive connected to AirPort Time Capsule: Partition not showing up

I connected an external USB hard drive to my AirPort Time Capsule. That hard drive has two partitions, but only one shows up in the Finder. It's the partition that I activated for Time Machine backups. I want to use the second partition for data.

How can I get both partitions of the external USB hard drive show up (mounted) when it's connected to the AirPort Time Capsule?

Both partitions have the same file system (Journaled). They do both show up when I connect the hard drive directly to my MacBook Air. But I'd rather access the partitions wirelessly, via the AirPort.