Coldcard MK4 microSD failure – works at first, then progressively stops reading cards (2 devices)

Bitcoin
1

I’m trying to see if anyone else has experienced this, or if this is a known edge case.

I’ve now had two Coldcard MK4 units show the same microSD failure pattern.

What happens:

  • MK4 works normally at first.
  • PSBT signing via microSD works for several transactions.
  • Then, suddenly, the MK4 can’t find PSBT files on the card.
  • After that, the problem gets worse:
    • The MK4 fails the microSD part of the device self-test
    • It can’t format cards
    • Even microSD cards that previously worked stop working on that device

This happened on my original MK4 and again on a replacement MK4.

Cards tested:

  • Multiple brand-new and known-good microSD cards
  • 8 GB and 32 GB
  • FAT32, MBR
  • Nothing exotic

Linux workflow (in case it matters):

  • Linux laptop with built-in SD card slot
  • microSD → SD adapter → laptop
  • Save PSBT file to the microSD drive
  • Right-click → Unmount
  • Wait for popup: “Filesystem has been disconnected”
  • Remove SD adapter, take out microSD
  • Insert microSD into MK4
    No unsafe removal, no write caching left active.

Why this seems odd:

  • Cards work initially, then stop
  • Failure is progressive
  • Same behavior across two MK4 units
  • Once triggered, there’s no obvious recovery path

This doesn’t feel like a simple “picky card” issue, because the cards do work at first.

2

I don’t have a MK4 myself, so I’ll let anyone else who has experience with them reply. I have the Q and haven’t seen this problem with it (though TBH I don’t do a lot of base layer transactions-- I mostly accumulate). From the description, my first thought is a mechanical issue (such as poorly soldered microSD card socket, or a design flaw that doesn’t properly manage the stress of the insert/remove action).