Time Machine won't change disk

[dwright1974]

Hi All

I have two external drives I use for Time Machine.

One I keep at home and use for my daily backups and one I keep at work and bring home once a month and use as an offsite backup.

Today I have brought my offsite disk home and Time Machine won't let me change disks!

When I click on the 'Change Disk' button in Time Machine Preferences, nothing happens!

I've tried repairing the (offsite) disk and that was fine. My 'daily' external disk will back up fine.

Can anybody help me fix why I can't change the disk?

Thanks in advance.

- D