Sorry to hear that. The solder joints on mine look fine.
I have a Samsung 32GB micro-SD card that has some test files on it. I loaded the card onto the new Teensy 4.0, and ran the program from the audio shield that tests reading 1-4 files at the same time.
The first time it ran, it stopped after reading the 2nd file. I power cycled it, and it sees a card is present, but it can't read the filesystem. I try several more times, doing things like re-inserting the card and reloading the program, and it similarly can't read the card.
Ok, I figured, maybe the file system got corrupted, so I load the card reader into one of my micro-SD readers, and mount it onto my Linux system, and the 4 files are there.
I then take it to another Teensy 4 that does have the micro-SD card attached, and I put the card in that reader, load up the program, and it can't read the filesystem.
I finally put the card into a Teensy 3.5, and load the program into the Teensy 3.5, where it works perfectly.
I will look to see if I have other cards and maybe reformat the Samsung card. I'm pretty sure I used the Samsung card when I was doing read speed tests in the January time frame.