Thanks for you comments. I downloaded the MMC specification 4.3 (JESD84-A43.pdf available at http://www.jedec.org) and it notes that SEND_EXT_CSD (CMD8)is not supported for cards which implements the standard earlier then 4.0 (Section A.8.2). So perhaps you have older card? Have you tried cards which implemented 4.0 or higher?
RE:
Hi Salim,
Thanks for you comments. I downloaded the MMC specification 4.3 (JESD84-A43.pdf available at http://www.jedec.org) and it notes that SEND_EXT_CSD (CMD8)is not supported for cards which implements the standard earlier then 4.0 (Section A.8.2). So perhaps you have older card? Have you tried cards which implemented 4.0 or higher?