HEy ben,
I had several problems with my H340 when using a SanDisk 400GB micro sd card in a CF to micro SD adapter card (looks exactly like this one shown here
https://groups.google.com/g/rc2014-z80/c/05GoFoi6gG8).
At some point I experienced something similar what you describe. In addition I found all my files on the sd card corrupted.
Just yesterday I spend more time to investigated what could be the problem and came across the following facts:
There are CF to SD adapters out there that support only up to 128 GB sd cards (LBA28). That is because of LBA that exists in different versions (
https://en.wikipedia.org/wiki/Logical_block_addressing).
Only CF to SD Adapters that are compatible with LBA48 can work with more than 128 GB of space.
Currently my CF to SD adapter is named as
FC-1307 SD to CF Ada USB Device in the Windows Device Manager. I have many more of those adapters but they are all the same type and not suited to work with SD cards larger than 128 GB.
The only thing that helped me, was to format the disk anew with FAT32...
Let us know if that helped you?