Basically, the problem is that geom disk list loves to hang, particularly with SD cards.
Symptom is that quite often with SD cards gpart show does not show the SD card.
geom disk list sometimes then shows the SD card correctly, making the SD card appear and ready for use, other times it just hangs.
When geom is hanging, gpart show also just hangs (and vice versa).
I had to remove a zfs pool that was imported for backup.
So I did that, not recognizing that when geom is hanging, access to disk hardware data seems to be blocked.
So zfs export somepool also got stuck.
Considering that likely zpool didn't write anything to the disks, I did poweroff.
I had to do a hard shutdown (long power button press) as the computer did not turn off itself at the end of shutdown.
After restarting, the ZFS pool showed okay, I exported it and removed disks.
Now I will have to try again reading my cameras' SD card. Let's hope this time it works (like it does most times), as otherwise I need to reboot again...
