I want all my songs that I have purchased available to download to a flash drive

Hello Alexandervalley,

If you are missing songs you can Redownload music - Apple Support onto your PC which I will paste below.

"How to redownload music in iTunes on your PC

Before you can redownload music that you purchased from the iTunes Store, you need to authorize your PC using your Apple ID.

1. Open iTunes. 
2. From the menu bar at the top of the iTunes window, choose Account > Purchased. If you use Family Sharing, you'll see Family Purchases instead of Purchased.
3. Choose Music from the menu in the upper-left corner of the iTunes window.
4. Find the music that you want to redownload. If you use Family Sharing, choose a family member next to Purchased to see their purchases.
5. Click the download button  next to a song or album.

Don't see the music that you purchased?"

Also, if this was the original device you purchased they may just have been moved outside of the iTunes Media Library folder. If so, you can consolidate them back to the iTunes Folder as copies. Once all are located you could then backup the Music Library folder to an external hard drive which I assume a flash drive would be eligible to be used.

All of those steps are outlined in the below Apple Support Article.

Back up and restore your iTunes library on your PC - Apple Support

I hope this helps.