[T-Mobile Samsung Galaxy S 4] Cynagen:
SKIP TO THE BOTTOM FOR THE MOST BASIC OF SCRIPTS TO ACCOMPLISH YOUR GOAL!

Long-winded, drawn-out-explanation:

It requires a whole new file system for a TRUE hybrid system which intelligent offloads the most used files to the fastest media available. You can do a JBOD RAID to do the same thing, but Android doesn't support RAID, and again, that's a "firmware" type thing where the RAID hardware/software translates your calls for sector 6689544 (doesn't exist in the 8gb usable space) to sector 2593544 of your SD card. There is no script that can accomplish this without maintaining an insanely large journal of where everything is located. I thought about this, but it would literally require a faux drive location, an extra service running on Android to listen for the system requesting files (presenting the faux drive location), and then making the appropriate call to the right drive. Wikipedia "File system"

Here's the basics of what happens when a file is requested by an application on ANY operating system:

1. File request to OS

2. File system opens journal (table/list) of files, finds match starting at sector 6689544.

3. File system opens appropriate drive location to retrieve sector 6689544, locks the sector and any other sectors with data for the file (prevent write and corruption of the data)

4. Read-ahead buffers data from subsequent sectors with continued data stream for the requested file

5. A. Returns either data stream if file is larger than buffer size, repeats steps 4 and 5 until finished; B. If smaller than max buffer size, entirety of the file

6. Releases lock on data sector(s) (to prevent corruption during read by an opposing write operation to the same location)

Now, that's just the basics, if you get to the nitty gritty, when you're addressing multiple volumes as one larger volume, this requires intersecting the request for data either at step 2 with replacing the file system, OR providing the original file system a faux "virtual" disk to address as mentioned on Wikipedia:

Either way, there has to be something to intelligently keep track of the location of the files, and with a Hybrid system like you're talking about, the best and fastest way would be a whole new file system. Creating a procfs "virtual" service is also possible, but is likely going to be much more prone to bugs and failures unless maintained very heavily. Porting an existing file system that can do this would be much much more feasible and can be built into the kernel directly. A simple script cannot accomplish this without just blindly mapping the SD card into the internal storage file structure.

Short and simple hack to accomplish what you want:

This is called a "symlink" which can already be accomplished quickly with the following command: "mkdir /storage/sdcard0/ext; ln -s /storage/sdcard1 /storage/sdcard0/ext"

There's the script you're talking about, but it's a dirty hack and yes, it'll work provided the "LN" command is available, but again, it's really dirty (I guess most don't care?)

If "mount" and subsequently "umount" are available commands in the Android terminal, something like this should work to completely remove the external SD mapping: (This is based on the Galaxy S II JB file system mappings:

umount /storage/sdcard1

mkdir /storage/sdcard0/ext

mount /dev/emmc /storage/sdcard0/ext

Now this is just a rough sketch of what you want to do, if you could provide me with a dump of a couple things, I can write up a script for you no problem. I'll need the following:

A listing of /dev: "ls /dev" output

The contents of /etc/fstab: "cat /etc/fstab" output

And the information on the "source folder" for the external SD card: "stat /External_SD" (or whatever folder it is)