You may loose applications that have been moved to SD card so it might be a good idea to move all installed apps to the phone memory.
Make sure your phone is rooted and your external SD card is FAT32 formatted.
Back up your original /etc/vold.fstab
Copy accross any data you want to be on new /mnt/sdcard from your /mnt/sdcard to /mnt/sdcard/external_sd
Unzip and put modified vold.fstab on you phone
In root explorer go to /etc
Re-mount the filesystem to R/W
Copy modified vold.fstab from wherever you have it to the /etc using root explorer
Change permissions to rw-r--r-- (644)
Re-mount filesystem as R/O
Reboot your phone
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