maybe you have it already unmounted , try proceeding , if it isn't unmounted , then reboot and try again , if rebooting doesn't solve it see my previous post for details , as I edited it .

[edit]
try un-mounting using :
umount /home/user/MyDocs
instead of umount /dev/mmcblk0p1 though I think there is no difference .

if you want to know if it's mounted or not then just type "mount" and nothing else , you will get a list of mounted partitions.