Can't backup external hard drive

Hey,

Ive got a macbook air, with two external hard drives. One for the backups and one for my iphotolibrary.

Time machine backups all the data of my macbook air (local). When i want to take the iphotolibrary hard drive off the list, where you can put hard drives and folders oni it, that you dont want to be backupped by time machine, it simply doesnt work. I cant select the iphotolibrary hard drive in the list, which makes it impossible to remove it.

With my mac, this worked without problem.

What do i have to do that time machine will also do a backup of the iphotolibrary hard drive?

The two hard drives are from lacie (1tb). My macbook air is running osx mavericks since one week.