I just installed FreeBSD 12 onto a Kingston 32GB SD card in a new apu4c4, BIOS version v4.6.4.
The installation process went fine, but several reboots later I see a flood of these messages on the console, starting as the kernel is configuring devices, and continuing until I shut the system down:
mmc0: CMD7 failed, RESULT: 1
mmc0: Card at relative address 1 failed to select
The system appears to work (presumably retries are succeeding), but it's a lot of messages and it's hard to feel good about deploying the system.
I thought it was odd that the installation went so well and then the problem started happening, so I powered the box off for several hours in case heat was involved, but apparently not.
Some people online reported that systems suddenly developed this problem when FreeBSD 11.2 was released, e.g., https://firstname.lastname@example.org/msg136078.html.
I can imagine that maybe I should try a BIOS upgrade, but it is not clear which version(s) I should try. I have read advice suggesting that FreeBSD users should not run the latest BIOS versions.
As a workaround I could try an mSATA device, but that seems like overkill for my application.