Boot.dat only mean the chip is working, so your problem is your SD card flex if there is no issue with the connector on the motherboard to the SD card flex.
This is so weird. I replaced the reader with the new again. Still no boot. But after pushing a little harder on the connector part, it boots again.
I'm afraid it's only temporary. But at least it works again
1710269585
Yeah it was a temporary...
So I've been using my modded V2 mariko switch with a hwfly chip, that has the sx firmware on it. When the other day it Suddenly won't read any SD cards anymore when booting. I've tried several to no avail.
So I've ordered a new sd card reader...