files disappearing on exFAT format USB hard drive

[Rocinante123]

I just spent several hours organizing files on an external USB HDD which is formatted with exfat. I noticed as I've been working that files I'm pretty sure I saw in there a moment ago suddenly are gone. There was no power outage, disconnection of the drive, or anything obvious that I can think of that might cause data loss.

This is happening on a PC running the latest Linux Mint. It's not a dual boot with Windows setup. I'd like to try and recover the missing files if possible so I don't have to retrace my steps and do all of the work over again starting from a backup. I have a backup which is exFAT (and which I'm therefore afraid to even start working with, for fear that the same thing might happen).

Any suggestions on how to proceed would be appreciated.