This error was documented when I received the Spectrum emulator card, now at least more than a year back.
I made by myself a bridge with two edge connectors and a flat cable, following the official schematics.
Once all connected I powered on the computer. I had some games on a directory, so first I did a CD to it. Then I invoked the emulator with :ZX.
Not knowing very much how it worked, I typed LOAD "" in Spectrum style...
At the prompt, I understood that instead a name, I could type an EXDOS command, so a :DIR or :BASIC could work.
With only a few lines the list of a DIR passed very fast. The selected game name loaded perfectly, but I'm not a gamer, so tried to load a second one. Then the first error happened, "File not found".
I tried another :DIR, but this time it returned garbage.
I typed :BASIC, and tried another :DIR from there. The same garbage.
I blamed my homely made bridge, and made some connection tests. Even I ordered a clone bridge from Pear. The same results.
Only that first time game load worked for me. You know I don't have a way to load from floppy, but I tested from Ramdisk and discovered the way to make the Emulator work.
Then I talked with Pear about another user problem like mine, but he didn't know more instances.
Then I bought another EP just to test this and another problems, just to discard my old EP, but my tests returned the same results.
Basically, accessing the SD from the Spectrum emulator corrupts its lecture, not the content, that is preserved. The corrupted lecture, once produced, persist on Basic, EXDOS or ISDOS. Only a hard reset fix it.
I have tried a number of different SD cards and formats, always the same.
It seems to me like a software error, but the first perfectly loaded game annoys me.
They aren't the games, only from where they are loaded.