Time Machine will not recognize external drive for backup

I have a large external drive that I have partitioned to contain Time Machine backups. However, when I open Time Machine and try to select the disk, all I see is "Other AirPort Time Capsule"; in other words, the drive I want to use is not visible.

I have been able to use this same drive (on a different partition) for Carbon Copy Cloner backups with no problems.

Any ideas?