UEFI firmware can't write to "fake" FAT hard disk

Using the Tianocore OVMF UEFI firmware, a UEFI application cannot write to the emulated fat disk (-hda fat:rw:path/here). A file will get created or written, but will be corrupted.

Looking through old bug tickets ... When reporting issues, please provide proper information on the versions that you were using (QEMU, OVMF, ...) and complete information about which command line parameters you used to start QEMU.

Out of scope; please see my (independent, more recent) replies here:

[edk2-devel] OVMF/QEMU shell based unit tests and writing to a virtual disk

https://edk2.groups.io/g/devel/message/66655
https://edk2.groups.io/g/devel/message/66656

(alternative links:
https://www.redhat.com/archives/edk2-devel-archive/2020-October/msg00877.html
https://www.redhat.com/archives/edk2-devel-archive/2020-October/msg00878.html
)