Cut and paste this URL to share the unmodified register and value:
https://regviz.com/r/Atmel/AT91SAM9M11/HSMCI0/SDCR#0x0
SDCBUS=1, SDCSEL=SLOTA
SD/SDIO Card Register
SDCard/SDIO Slot
0 (SLOTA): Slot A is selected.
1 (SLOTB): -
2 (SLOTC): -
3 (SLOTD): -
SDCard/SDIO Bus Width
0 (1): 1 bit
1 (4): 4 bit
2 (8): 8 bit
https://github.com/cmsis-svd/cmsis-svd-data