booting error on pandaboard

when i plug sdcard into the board , i got the following errors :

U-Boot SPL 2013.01.-rc1-00003-g43ee87a (Jul 22 2013 - 05:47:23)

OMAP4460 ES1.1

SDRAM: identified size not same as expected size identified: 0 expected: 40000000 OMAP SD/MMC: -1901534549

spl: mmc init failed: err - -19 ### ERROR ### Please RESET the board ###

I did exactly as this link says. http://releases.linaro.org/13.07/android/panda/#tabs-2

I guess the error caused by the following step:

linaro-android-media-create --mmc /dev/sdc --dev panda --boot boot.tar.bz2 --system system.tar.bz2 --userdata userdata.tar.bz2

however,I do not know how to solve it ,can you give me some suggestions? thank you in advance!