we use MQX 4.1 on K63. After systemrestart file access on SD_Card is correct. But after a few write calls the transferrate ist very very slow. Even a 'dir' gives only one line per 5 seconds. This is not on USB devices, so I think it must be a esdhc driver problem. The older hardware based on K60 running with MQX 4.0.2 doesn't have that problem.