Acer has created a pretty nifty mechanism with the Storage Expansion, which fuses an SD card with the internal SSD. It's also possible to simply upgrade the SD to a higher capacity one without losing any data. The magic behind most of this is aufs.

There are two real, physical if you will, directories involved and one virtual directory. The first physical directory is the home directory on the SSD, /home/user. The second physical directory is the SD, /media/disk. aufs unifies both directories (branches) into a virtual directory, /mnt/home. Imagine the virtual directory like an additional layer on top of both branches. If one branch dissapears then it still works, the files are just not available in the virtual directory anymore.

The beauty of it is that each branch can still be written to individually, with all files automatically showing up in the virtual directory. The interesting part begins when files are directly written to the virtual directory. aufs will then distribute the files among all branches based on the selected policy. There are three major policies available: tdp writes to the highest branch first if available. rr distributes all files evenly among all branches using a round-robin mechanism. mfs writes to the branch with most free space first.

The policy used on the AA1 is mfs. As a consequence the SD will only ever be written to if the SSD has less free space available, which will be the case with 8GB and 16GB cards most of the time. Still it'd probably be a better idea to at least use rr, to distribute it evenly. The best solution is to prioritize the SD, especially if you have a fast SDHC card like the SanDisk III.

Using the tdp policy and the SD as the highest branch will always write to the SD card first until it runs out of space. That's probably a good idea, considering that it only concerns the virtual home directory. The SSD will be busy enough with all the other file operations on the system, so why not lift some work of it. The virtual home directory is used by default by the file manager and as the download directory by Firefox, probably by a few other applications as well.

It is fairly easy to do, in fact the line below automates the whole process.

sudo sed -e'63s/mfs/tdp/;99s/append/prepend/' -i.1 /etc/aufs.act

It becomes effective after a reboot.

To check if and how aufs distributes the files among both branches use df -h on the terminal, which will show the available and used space for each mount point.