FolderMount is a Android Application released XDA senior member madmack available for all rooted Android Devices ( Support for Android 2.3 – 4.2.2 ) . As mentioned in the post title this application links folders from internal SD to external SD to free space.
But why you need to install FolderMount ? See below note developer at XDA
“Today some apps store big files over the internal SD card (e.g. Games, Podcasts, Offline RSS feeds, Pictures, Offline navigation software). Typically users have a small internal sd card size (~16GB) while the external SD is a lot bigger (~32-64GB). Most of these apps don’t have an option to use the external SD card to store data so they starve your internal SD card memory. I’m bringing this solution to fix this issue. In my case, I use this app to mount offline navigation software and some Game Loft games (NFS anyone?).”
There are two versions of this application available. free and premium. Users who buy premium key will have more features like Unlimited folder pairs (free version has a limit of 3) and several others.
This is a great application serving the need for more storage on devices with small internal SD size ( ~ 4-8 GB ). Check more about the features, screenshots on how to link the folders , change log and download link here at XDA.