I've known SD cards to have issues with the contacts on the sockets. They can get dirty or greasy, the springs may need to be retensioned too. This can make them intermittent. Buff with a pencil eraser if possible, clean with an alcohol wipe. I had an RPI model B (big SD card) and the socket fell apart.
Thanks for all the hints. I was unable to do anything in my shell
window, so I power cycled. boot halted with fsck/journal issues.
After a previous problem, I had swapped the card, but the other one had
trouble too. On the original one (back to using it now), I wrote every
sector and read it back and it really seemed ok.
So this seems most like Brad's i2c lockup, except this machine for
hardware has
USB wifi (urtwn0)
USB serial (uplcom) to talk to weather station
(no keyboard, display)
and it runs
usual server background daemons, plus tor
weewx
Putting the uSD in a reader in a netbsd-8 amd64 machine, I got:
$ fsck /dev/rsd4a
** /dev/rsd4a
Unrecognized wapbl type: 0x00000000
UNABLE TO READ JOURNAL FOR REPLAY
CONTINUE? [yn] y
CLEARING EXISTING JOURNAL
** Last Mounted on /
** Phase 1 - Check Blocks and Sizes
** Phase 2 - Check Pathnames
** Phase 3 - Check Connectivity
** Phase 4 - Check Reference Counts
** Phase 5 - Check Cyl groups
59696 files, 1049597 used, 14146329 free (1849 frags, 1768060 blocks, 0.0% fragmentation)
MARK FILE SYSTEM CLEAN? [yn] y
and then I mounted it and my sqlite db for weewx (weather station)
checked out.
I then used dd to read the whole card, which worked fine. With bs=1m,
and a USB reader, I got
31914983424 bytes transferred in 1224.449 secs (26064771 bytes/sec)
which means there was no trouble.
With the bad journal cleared, and the card reinserted, the machine boots up normally.