[Samsung Galaxy Ace Plus S7500] Inny:
This didn't work for me. It appears my S7500 isn't executing the script in /etc/init.d.

So I'll just leave the method I cooked up (obviously based on OP's script) here, which did work for me:

/etc/init.d/00remountextsd

#!/system/bin/sh

###########################################################
# Remount external SD as internal storage #
###########################################################
# Place in /etc/init.d and reboot #
# If that doesn't work, use SManager App #
# - Open app, find this script file #
# - Enable by clicking 'Su' and 'Boot' icons and hit Save #
# - Reboot your phone again.. and enjoy. #
###########################################################

# ..\Generic\adb push 00remountextsd /storage/sdcard0/
# mount -o rw,remount /system;cp /storage/sdcard0/00remountextsd /etc/init.d/&&rm /storage/sdcard0/00remountextsd
# mount -o rw,remount /system;cp /data/log/00remountextsd /etc/init.d/&&rm /data/log/00remountextsd

LOG=/data/log/remountextsd.log
MOUNTOPTS=rw,dirsync,nosuid,nodev,noexec,uid=1023,gid=1023,umask=0007
INTERNAL=/dev/block/vold/179:23
EXTSD=/dev/block/vold/179:97

echo -n "Starting $0, " | tee $LOG
date | tee -a $LOG
#echo "---------------------------------" | tee -a $LOG
#echo Waiting for /mnt/media_rw/sdcard1 to come online... | tee -a $LOG
#for i in $(seq 1 1 3000)
#do
# if ( grep -c "/mnt/media_rw/sdcard1" /proc/mounts ); then
# break;
# fi
# usleep 100000
#done
#date | tee -a $LOG
#echo "---------------------------------" | tee -a $LOG
#echo Waiting for /mnt/media_rw/sdcard0 to come online... | tee -a $LOG
#for i in $(seq 1 1 3000)
#do
# if ( grep -c "/mnt/media_rw/sdcard0" /proc/mounts ); then
# break;
# fi
# usleep 100000
#done
#date | tee -a $LOG
echo "---------------------------------" | tee -a $LOG
echo "Current mounts:" | tee -a $LOG
cat /proc/mounts | tee -a $LOG
echo "---------------------------------" | tee -a $LOG
echo "Current disk utilisation:" | tee -a $LOG
df | tee -a $LOG
echo "---------------------------------" | tee -a $LOG
echo "Kernel information:" | tee -a $LOG
cat /proc/version | tee -a $LOG
echo "---------------------------------" | tee -a $LOG
echo "Umounting.." | tee -a $LOG
if mountpoint -q "/mnt/media_rw/sdcard1/.android_secure"; then umount /mnt/media_rw/sdcard1/.android_secure 2>&1 | tee -a $LOG; fi
if mountpoint -q "/mnt/secure/asec"; then umount /mnt/secure/asec 2>&1 | tee -a $LOG; fi
if mountpoint -q "/mnt/secure/asec"; then umount /mnt/secure/asec 2>&1 | tee -a $LOG; fi
if mountpoint -q "/mnt/media_rw/sdcard0"; then umount /mnt/media_rw/sdcard0 2>&1 | tee -a $LOG; fi
if mountpoint -q "/mnt/media_rw/sdcard1"; then umount /mnt/media_rw/sdcard1 2>&1 | tee -a $LOG; fi
echo "Mounting sdcard0 (extSD)" | tee -a $LOG
mount -t vfat -o $MOUNTOPTS $EXTSD /mnt/media_rw/sdcard0 2>&1 | tee -a $LOG
sdcard /mnt/media_rw/sdcard0 /storage/sdcard0 1023 1023 &
echo "Mounting sdcard1 (internal storage)" | tee -a $LOG
mount -t vfat -o $MOUNTOPTS $INTERNAL /mnt/media_rw/sdcard1 2>&1 | tee -a $LOG
sdcard /mnt/media_rw/sdcard1 /storage/sdcard1 1023 1023 &
echo "Mounting .android_secure" | tee -a $LOG
mount -t vfat -o $MOUNTOPTS $EXTSD /mnt/media_rw/sdcard1/.android_secure 2>&1 | tee -a $LOG
echo "Mounting asec" | tee -a $LOG
# Delay for half a second for device to become available
usleep 500000
mount -t vfat -o $MOUNTOPTS $EXTSD /mnt/secure/asec 2>&1 | tee -a $LOG
echo "---------------------------------" | tee -a $LOG
echo "Final mounts:" | tee -a $LOG
cat /proc/mounts | tee -a $LOG
echo "---------------------------------" | tee -a $LOG
echo -n "Finished running $0, " | tee -a $LOG
date | tee -a $LOG

Thanks and enjoy!

[EDIT]: An alternative to using the SManager app would be most welcome!

[EDIT2]: Running this mod as-is, "FIFA15" crashes after start. If I use "Use external SD as primary" from Settings->Storage (not available on all ROMs), it does work, but then the external SD is not available over MTP...

---------- Post added at 19:25 ---------- Previous post was at 19:22 ----------

[MENTION=4193750]KaylordTheuZ[/MENTION]

You understand correctly.