Did you tell your computer to remove the hardware from the USB before pulling it out? Yanking it out prematurely can corrupt the file. Also, is the file a .BIN?

My GoT Prem did not recognize the first 2 USB thumb drives I tried. Sounds like you got farther than this.

Did you try a fresh download?

I had this happen too, if two or more sticks fail the SD card is having some problems. I called STERN and had a new card shipped to me with the newest code already installed. It only took about 3 days to get to me, and the best thing you can still play the game with the old code and card until it arrives.

Does that ".spk" mean SPIKE? I wasn't aware that there was a difference between the systems?

Sounds like you'll need a fresh SD card pre-loaded.

I don't have a SPIKE game yet but for my SAM games, I've have had many cases where certain thumbdrives didn't agree with the machine for whatever reason. (Goldie Locks - Too Big, Too Small, wrong file system, etc)

When that new SD card comes in, it may be worthwhile to make an image of it for personal backup reasons.
(Create an Image / ISO from the SD Card and save to your computer.)

If you are on a mac, Disk Utility will do it. On a PC, PowerISO is a good solution, I'm sure there are others as well.

Another thread on that if it helps: https://pinside.com/pinball/forum/topic/cloning-duplicating-stern-spike-sd-card-on-newer-stern-pinball

Good luck- hope your epic game gets resolved soon!