MultiMediaCard Protocol - USB Mass Storage: Designing and Programming Devices and Embedded Hosts - page 96

R1b
Response type R1b is identical to the R1 response format except the
response byte is followed by one or more bytes that indicate busy status. The
card returns zeroes continuously while busy and returns a non-zero byte
when ready to receive another command. In other words, DataOut remains
low to indicate busy and goes high when no longer busy.
R2
In SPI mode, response type R2 is used only for the SEND_STATUS com-
mand. The response is two bytes. The first byte is identical to the R1 for-
mat. The second byte contains eight status bits. On error, one or more of
the bits are set:
Bit 0: The user has locked the card.
Bit 1: The host has attempted to erase a write-protected block or a
sequence or password error occurred during a card lock/unlock operation.
Bit 2: General or unknown error.
Bit 3: Internal card-controller error.
Bit 4: A card's internal error-correction code was applied but failed to cor-
rect the data.
Bit 5: The command tried to write to a write-protected block.
Bit 6: The selection of erase groups was invalid.
Bit 7: The command argument was out of range, or the command is
attempting to change the ROM section, or the command is attempting to
reverse the copy bit or permanent write-protect bit in the CSD register.