The Galaxy Tablet 7.0 Plus does not be default allow third party File management application write to the external sdcard. This is a permissions issue. The solution found at the xda developers forum is:
Simple solution for external sdcard write access on Android 3.2…if you’re rooted
I’ve found a simple solution for gaining write access to the external sdcard (/mnt/sdcard2) on Android 3.2. The sdcard is mounted r/w, but the owning group is media_rw which is linked to the WRITE_MEDIA_STORAGE permission. Unfortunately, this permission was not required for access to the external sdcard prior to 3.2 so most file management utilities do not request it when they run…meaning they only have read access to the card.
The simple solution, if you’re rooted, is as follows:
1. Using a root-enabled file manager, navigate to /system/etc/permissions and make sure you have mounted the file system r/w.
2. Edit platform.xml and find the WRITE_EXTERNAL_STORAGE permission.
3. Add an additional group definition for this permission…4. Save the changes to the file.
5. Restart your tablet.After this change, any apps that request the WRITE_EXTERNAL_STORAGE permission (which includes almost anything that access either of the sdcards) will gain group membership in both sdcard_rw AND media_rw, giving them r/w access to both sdcard filesystems.
To mount the filesystem read only use mount -o rw,remount -t yaffs2 /dev/block/mtdblock3 /system
I found an easy solution was to copy the platform.xml file to somewhere with easy access from the PC, edit the file on the PC and copy it back to the system location with a su-ed shell.