"Play More Like This" feature stopped working

Some time over the last few months the feature to "Play More Like This" music on the Toyota Prius C audio system stopped working.  The system displays a message "No similar songs found" even though it has found songs in the past...since there are over 8,0000 songs on the drive it can't possibly not find similar songs!

{A little background: my music is stored on a 32 gig thumb drive plugged into the USB port of the car audio system. The audio system allows you to browse and play music stored on the drive.}

"Play More Like This"  feature allows you to ask the system to search your music collection for similar songs and create  a list of songs to play. This is a great feature since you can get a new playlist with only hitting a single button rather than hitting multiple buttons to browse for songs....safer driving!

So what is the problem?

I'm on the trail to finding out what's wrong, but no answer at this point.

Here's what I've done so far:

• Searched the internet for postings about the problem. It seemed that few people have encountered this problem, and I could not get any clues to a solution.
• My suspicion is that there is some error on the drive causing the feature to return no songs to the "Play More Like This" search.  
• I plugged the thumb drive into my computer and ran "Error Checking"  on it. No errors were reported.
• I plugged another thumb drive into the USB port of the car with a sample music selection and tested the feature. IT WORKED! From this test I conclude that the car system is working and the thumb drive is the problem. But how to find it.....
• So I plan to delete files {say the newest ones from the last 6 months}  from the thumb drive and test in order to find the files that may be causing the problem...This is time consuming test since it takes time for the audio system in the car to read the contents of the drive and be ready for the "Play More Like This"  feature to be invoked. Like waiting for a computer to boot! Also I need some method the delete finds from the thumb drive without going back and forth from my car to desktop computer....e.g. a laptop or tablet.

So on to the testing, and I'll report back soon.

7/9/2013:

Deleting files did not help to isolate the problem, "Play More Like This" still would not work.

I also tried a program called "Mp3checker" to check the tags and format of the files on the USB drive.

Several files were bad according to the program. I removed these bad files, but "Play More Like This" still did not work.

Interesting though, I found some documentation that states the limitations of the USB music device; e.g. number of files, number of directories, etc.  I thought this pointed to a cause of my problems...
Limits are:

• Max directory levels:8
• Max number of folders:999
• Max number of files: 9999
• Max number of files per folder: 255

and then out of nowhere "Play More Like This" would work at times! So my testing so far as not isolated the problem...Oh well.. so still working on this problem.