Time Machine Backup disk not mounting?

Upgraded to Lion over a month ago on my MBPro
In Time machine I made an encrypted back up to an external HD via USB 2.
External drive has 2 partitions. 'Media', for pics and stuff, and 'Backup'.
Connected the external HD to my airport extreme using the USB 2.
My 'Media' partition mounts, and is visible in my finder over my wireless network. I've used this setup over 2 years.
However, now the 'Backup' disk is not visible on my wireless network. I can only 'see' the backup partition if I connect via the USB wire.

!?

Any suggestions appreciated.
Ones that help me resolve this Especially so.

Try Finder and look under shared locate backup and double click it should re-alias and then open Disk Utility and select TM backup or what ever name you gave it then it should start a countdown from 120 seconds but if you close system preferences it will start immediately.