The sdcard can be umounted by using the umount command.
how can it be remounted again . I tried using the mount command
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Connect your device and at the command prompt:
adb devices & press enter (Your device should now be listed)...
adb shell & press enter (Wait for the "#" to appear)....
mount sdcard next to the "#" sign & press enter...
You will need to know the mount point also if you intend to mount the external card.
As an example:
su mount -o bind /mnt/emmc /sdcard/external_sd