trouble backing up external drive with time machine
I have a small USB stick that I use to store important documents but documents that I don't access very often. I have an additional 500 GB hard drive that I use for backing up with Time Machine. I want Time Machine to backup both my internal hard drive (Macbook Air 2011 Lion) as well as the small USB stick. I've searched the forum and discovered that I needed to reformat the USB stick, which I did (it is now Mac OS Extended). The USB now appears in the list of excluded drives in Time Machine preferences, which I removed. Thinking that Time Machine would be backing up my USB stick now, I backed up and then turned the computer off. The next day, I looked at the external drive, and it didn't seem to have a backup of the USB. I entered Time Machine and the USB is "greyed out" which means it wasn't backed up. I've retried backing up several times by including the USB in Time Machien preferences, but it seems that when I click "save" it doesn't actually save because when I open Preferences up again, the USB stick is in the list of excluded items again. Have I done something wrong? Any imput would be greatly appreciated.
MacBook Air, Mac OS X (10.7.5)