EMX fails to write to microSD

Working through a backlog of faulty boards, I’ve got one that will happily read from its microSD card, but fails to write.
I’ve tried with different microSD cards. Tried the microSDs in identical boards, running identical firmware.
Looks like it has to be hardware. uSD supply is direct off the 3.3V rail, so it isn’t the problem of low supply from the EMX we saw years ago.
I don’t know anything about the protocol the EMX (SDK 4.2) uses to drive the SD cards, and I can’t see a line labelled Write.
Is there a specific line that could be faulty such that reads work but writes fail, or should I be looking elsewhere?

I first picked it up when changing settings (LED brightness, LCD bias voltage, etc). These are stored both in ExtendedWeakReference and in a file on the uSD card, FactoryDefaults.csv. The file overrides if it is present.
LCD Bias voltage is particularly important, it sets the analog out level to the LCD bias voltage and if wrong you can’t read the display.
Well, I set it to a good value. Rebooted, and blow-me-down, can’t see the display!
Tried again. Same result.
Looked at the uSD in a PC card reader. The data isn’t being updated, the file is unchanged. It has failed to write!
No problem reading, though. So I swapped uSD cards, I have stacks - problem stays with the specific PCB.
Try the uSD cards on other PCBs with identical firmware. They work fine.
Trace the file write operation in the debugger. System thinks it works, but it doesn’t.
Sounds like hardware, is there a line stuck? Hmm, which line to look at? Depends on the protocol used, and that isn’t disclosed.
Is it even possible a stuck line could cause this problem with the uSD protocol in use? I don’t know, I’ll ask someone who might.

As previously mentioned, I’m just working through some failed boards. A some that were new and failed, some that have come back in after years of use. This is a new one. A couple of other EMX modules are dead, one works perfectly except for USB, another loads ok but seems to have a flakey memory area. If I can find a quick fix for this one good, else it goes in the bin.

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@ C-Born - Have you tried to reformat the SD cards that fail?

@ John - I’ve swapped around microSD cards and main boards such that I’ve isolated the fault to this board. I was hoping that someone who knew which microSD protocol the EMX/NETMF was using could say whether a single line failure could lead to the symptoms of a card being fully readable but writes not working at all.
I made up a simple breakout to check power supply to the socket, which seems fine.
I’ve just ordered a full microSD breakout board so I can monitor it with a logic analyzer.
Probably a bad line, a solder bridge or something.
Not a big issue, in my cleanup I’ve found several more failed EMXs with various problems (USB doesn’t work, faulty memory, fails at high temperature etc) but it is probably more economical to chuck them rather than spend more time trying to fault-find.