I was playing with the microSD card extender from Pine64. It seems to have damaged the socket. There is a card detect switch built into the socket and it no longer functions. This causes U-Boot to hang if there is a bootable SD card in the socket. It tries to boot from it and then fails the card detection, even though it is running from the card itself.
In the serial debug output it shows:
Trying to boot from MMC1 MMC: no card present spl: mmc init failed with error: -123 SPL: failed to boot from all boot devices ### ERROR ### Please RESET the board ###
Another PinePhone user had the same issue https://forum.pine64.org/showthread.php?tid=13775
To fix the problem I soldered the switch pin on the socket to ground. When a card is inserted it would normally short the pin to ground (T6 is connected to ground). It now thinks there is always a card inserted. It does not seem to cause any problems.
I actually soldered the pin to the metal shield since it is connected to ground.
If you think you have a broken card detect switch you can try booting off this modified JumpDrive image. It ignores the state of the switch. If it boots and the normal JumpDrive does not, then you have a broken switch.