Asking the obvious, have you rebooted the printer (I'm sure you likely have), tried reloading Chitubox (though that would not explain the original flash drive not working).? Is the flash drive readable in you computer, can you perform a file system integrity check on it? it is common for flash drive file systems to get corrupted when swapped between devices, especially if not properly dismounted (wait for ALL activity to stop before removing it by watching it's activity light, do not trust the computer saying it's OK). My LInux system says it's OK just after clicking "Eject", however the drive's activity indicator will blink for 2-3 seconds after that.

I take it that sliced files that did at one time print now do not? If not this is likely not a Chitubox issue,but rather a FlashForge problem--have you contacted them?

Hello, I think you should check the flash drive. Connect the flash drive to a computer and see if it's readable. If it is, you can perform a file system integrity check on it to ensure that it's not corrupted. On Windows, you can right-click on the drive, select "Properties," go to the "Tools" tab, and click on "Check" under the "Error-checking" section. On Linux, you can use the fsck command with the appropriate options for the file system type. Hope that geometry dash scratch is useful for you