I didn't want to keep using my iPod in the car, so I sat down with my Apple PowerBook and dragged some songs from iTunes onto a USB flash keychain or thumb drive.
During this first attempt, some played, some did not, and I would see "UNPLAYABLE FILE" between every song.
Some searching on FitFreak turned up a couple of posts where people had the answers to this problem (what a knowledgeable following the Fit has).
1. Songs purchased on iTunes won't play in the Fit from anything other than an iPod to which they're associated.
2. Having a Mac means that your operating system likes to spread itself around: little files you cant see are placed on the thumb drive by the operating system, and the radio in the Fit spits them out as "UNPLAYABLE FILE" between every song it can play.
So today I got a new larger thumb drive (8GB), and went into iTunes to drag some songs to it. I enabled the "Kind" column under the View>View Options menu. Then I sorted the songs so all the Protected AAC files were grouped toward the bottom. Then I made some new folders on the thumb drive - one for each artist. Then I dragged the songs I liked from each artist into the folders. This way in the car I could use the big knob on the radio to scroll through artist folders and pick one that way.
Then I wanted to stop the hidden files from being hidden, but found that even if I could, resolving the problem that way would be more trouble than I cared to undertake. Then I found this site:
BlueHarvest Hidden File Cleaner Upper
Well worth looking into. It took 788 files off my thumb drive that would have amounted to that many instances of UNPLAYABLE FILE showing up.
Considering I commute 30 minutes each way, I've got more than enough music ripped from CDs or from the old Napster days for the ride, but I still want to listen to stuff I bought off of iTunes, so now I'm looking into stripping the DRM away from all the $.99 songs I've purchased so that I can enjoy them in the car without my iPod Touch which is usually on my person and with me at work for other usage.