[Samsung Epic 4G Touch] egingell:
I have successfully mounted my internal SD card to /data/sdext2, mounted my external SD card to /storage/sdcard0, and bound /storage/sdcard0 to /storage/extSdCard (for backward compatibility) using the following scripts.

[size=+1]I provide this information with no warranties or guarantees and I only tested it on the Sprint Epic 4G Touch with stock rooted Jelly Bean 4.1.2. Use it at your own risk.[/size]

Requires:

root

busybox

basic knowledge of shell scripts and Linux (not necessarily required, but it doesn't hurt)

Notes:

Make sure /system/etc/init.d/12binds, /system/bin/debuggerd, and /system/bin/debuggerd.bin are executable. Just to be safe, you can execute this from a terminal emulator (the asterisk will make it get multiple files at the same time).

mount -o remount,rw /system
chmod 755 /system/bin/debuggerd*
chmod 755 /system/etc/init.d/12binds
mount -o remount,ro /system

The reason I converted my SD card to EXT4 is so I can move&bind internal app data (the stuff in /data/data/app.package.name) to /data/sdext2/data/app.package.name and preserve file permissions. exFAT does not support this.

Terminal emulator - Executing this command will wipe the SD card and convert it to EXT4. This is optional:

mkfs.ext2 /dev/block/mmcblk0p11

/system/etc/vold.fstab

[color=green]## Vold 2.0 Generic fstab
## - San Mehat (san@android.com)
##

#######################
## Regular device mount
##
## Format: dev_mount <label> <mount_point> <part> <sysfs_path1...>
## label - Label for the volume
## mount_point - Where the volume will be mounted
## part - Partition # (1 based), or 'auto' for first usable partition.
## <sysfs_path> - List of sysfs paths to source devices
## storage_struct - ex) series, "/mnt/sdcard/extStorages" / parallel
######################
# internal sdcard[/color]
{
ums_sys_path = /sys/class/android_usb/f_mass_storage/lun0/file
secure_format = enable
discard = enable
}
[color=green]#dev_mount sdcard /storage/sdcard0 11 /devices/platform/dw_mmc/mmc_host/mmc0/mmc0 encryptable_nonremovable[/color]
dev_mount sdcard /storage/sdcard0 1 /devices/virtual/block/cyasblkdevblk0

[color=green]# external sdcard[/color]
{
ums_sys_path = /sys/class/android_usb/f_mass_storage/lun1/file
android_secure_containers = enable
}
[color=green]# This partition is 300MB (the smallest I could make it using MiniTool Partition
# Widard) and is [b]not[/b] set to active. It does not mount, which is how I want it. I know
# I'm wasting 300MB of disk space, but that's ok by me.
# This line is here to prevent "damaged SD card" errors.[/color]
dev_mount sdcard1 /storage/extSdCard 2 /devices/virtual/block/cyasblkdevblk0

/system/bin/debuggerd - Make sure you rename the original to /system/bin/debuggerd.bin if you haven't already done so.

[color=green]#!/system/bin/sh[/color]
LOG=/data/link2sd-debuggerd.log
echo "$(date) mounting..." > $LOG

[color=green]# Delete or comment out this line if you want to make your external SD card into
# an internal and external SD card (both partitions set to active).
# I have not tried setting both partitions on my external SD card to active, so
# I don't know if they will both mount.
# This line binds the internal SD card's directory to the external SD card's
# directory. In other words both internal and external SD cards will be
# the same physical media. You will have duplicates in Music and Gallery.
# If you are not concerned with backward compatibility, you
# can delete or comment out this line.[/color]
mount -o bind /storage/sdcard0 /storage/extSdCard 1>>$LOG 2>>$LOG

[color=green]# Change "ext4" to "vfat" if you did not convert your internal SD card to EXT4.[/color]
mount -t ext4 -o rw,noatime,barrier=0,nobh,errors=continue /dev/block/mmcblk0p11 /data/sdext2 1>>$LOG 2>>$LOG

echo "" >> $LOG
echo "init.d" >> $LOG

[color=green]# Leave this line in if you want init.d support.[/color]
busybox run-parts /system/etc/init.d 1>>$LOG 2>>$LOG

echo "" >> $LOG

mount >> $LOG
echo "$(date) mount finished" >> $LOG
echo debuggerd.bin launched >> $LOG
exec /system/bin/debuggerd.bin

/system/etc/init.d/12binds - This is optional and requires more work than you might think and requires your internal card be EXT4, exFAT will not work. It also requires the "run-parts" line in /system/bin/debuggerd.

[color=green]#!/system/bin/sh[/color]
LOG=/data/symlink.log;
echo "Symlink" > $LOG

busybox mount -o rw,remount /system
busybox mount -o rw,remount /

cd /data/sdext2/data/

for APP in *; do
cd /data/sdext2/data/$APP;
for DIR in *; do
if [ $DIR != "lib" ]; then # don't bind the lib folder, Link2SD does stuff with it.
echo "" 1>>$LOG 2>>$LOG;
echo "Binding /data/sdext2/data/$APP/$DIR => /data/data/$APP/$DIR" 1>>$LOG 2>>$LOG;
mount -o bind "/data/sdext2/data/$APP/$DIR" "/data/data/$APP/$DIR" 1>>$LOG 2>>$LOG;
fi
done
done

busybox mount -o ro,remount /system
busybox mount -o ro,remount /

Make sure it is executable (chmod 755 /system/etc/init.d/12binds)

To make the above work:

If this is too much for you, don't do it.

Copy /data/data/app.package.name to /data/sdext2/data or if the folder already exists in /data/sdext2/data, copy the contents of /data/data/app.package.name to /data/sdext2/data/app.package.name. Use Root Explorer or any other root file explorer that will preserve the owner of the files and their permissions. Do not use the "move" option or you will be making new folders and trying to figure out what the owner and group should be!

Open each folder except lib in /data/data/app.package.name and delete everything in them.

Execute /system/etc/init.d/12binds or reboot.