Something interesting happened.
I put the sd card in my PC to format, and it showed two drives: D (which it calls "USB Drive") and E (SDXC). D is 448gb in size; E is 29.1gb.
When I installed the new sd card into the Switch, I used hekate to do something to it; I just can't remember what. (I have health-related memory problems.) I do remember that I created an emunand (I was clueless when I first hacked my Switch, and used the sysnand for atmosphere). I also have a vague memory of creating a partition in this sd card - whether it was to backup something on the Switch, I don't know.
So it seems like the partition was what was failing. I opened up the D drive, and it had atmosphere-related folders in it. (I formatted both drives D and E though, so now it's empty.)
Anyway, here is the H2Testw result:
Test finished without errors.
You can now delete the test files *.h2w or verify them again.
Writing speed: 72.6 MByte/s
Reading speed: 83.2 MByte/s
H2testw v1.4
That is the result on E. I still need to test D. Thank you for helping me.