I purchased two new USB drives and used the Unraid USB Creator application to install a recent backup of my existing flash drive. As suggested, I powered down my Unraid server and inserted the existing USB drive into my Windows PC and ran Error Checking. After a minute or so, it reported no errors were found.
I did notice that the slot on the motherboard that I had been using since day one without issue was USB3, and I thought I remembered reading somewhere that could cause issues. I plugged the existing USB drive into a USB2 port, fired up Unraid and re-ran the Fix Common Problems plugin. No more bread errors on the USB drive. I did recently upgrade to the current version of Unraid so maybe a new driver caused a hiccup.
Thankfully I had a recent USB backup and two brand new USB drives but I'm glad I didn't need either of them.
@trurl Thank you for your advice!