It seems that some cards do not tolerate releasing the card (by setting CS
high) after issuing CMD17 (and 18) and raising it again before reading
data. Somehow this causes the 0xfe data start marker not being read and
SDCard.readinto() is spinning forever (or until this byte is in the data).
This seems to fix weird behviour of SDCard.readblocks() returning different
data, also solved hanging os.mount() for my case with a 16GB Infineon card.
This stackexchange answer gives more context:
https://electronics.stackexchange.com/questions/307214/sd-card-spi-interface-issue-read-operation-returns-0x3f-0xff-instead-of-0x7f-0#307268
This commit fixes two things:
1. Do not allocate on the heap in readblocks() - unless the block size
is bigger than 512 bytes.
2. Raise an error instead of returning 1 to indicate an error: the FAT
block device layer does not check the return value. And other
backends (e.g. esp32 blockdev) also raise an error instead of
returning non-zero.
Peter Hinch