Why is my "Encrypt backups" option greyed out for local (USB) time machine disc? I'm using Mountain Lion on a new MacBook Pro (retina) with a local USB drive for backup. From what I've read searching around (including other questions on here) I should be able to encrypt my time machine backup on this local USB drive, but the option is greyed out.

The USB drive is 500.11 GB in two partitions of 251.33 and 248.77 GB - the first partition is the time machine backup drive, formatted as Mac OS Extended (Case-sensitive, Journaled). My Mac's hard drive is internal SSD 250.14 GB formatted as Mac OS Extended (Journaled). I don't know what this means, but the internal SSD and the first partition of the USB drive (time machine backup) both say YES for Owners Enabled and the second partition on the USB drive says NO.

Does any of this explain why "Encrypted backups" would be greyed out?

What would I need to do to fix it?
Make sure that your partition scheme is GUID, not Master Boot Record (MBR) or Apple Partition Map.

To enable encryption, the partition map must be changed to GUID. Note that this will make your drive invisible to older PCs and will wipe all of the data on the drive.