Hello!

Mounter is an API that is there to provide mounting-like utilities.

Features:

mounting - mount any directory to any other. (For example, mount a disk drive to /borkbork/ or so)
unmounting - reversing the above step
chrooting - changing the root directory to something else (mounting still works, this is useful in combination with chrooting when you want to mount a directory that is "below" the chrooted one)
unchrooting - reversing the above step

And yes, this API is sandboxing compatible. I used it for PBL: http://www.computercraft.info/forums2/index.php?/topic/29826-pbl-the-partition-bootloader/

Installation
SpoilerTwo ways:
1. It is bundled with PBL (see link above)

2. Install it via EMPaM:

2.1 get EMPaM: http://www.computercraft.info/forums2/index.php?/topic/29811-empam-extensible-modular-package-manager-v010/page__pid__278960#entry278960
2.2 run empam –addrepo mrobsidy-main https://github.com/MrObsidy/technical-data-inf/blob/master/empam/mrobsidy-main.repo
2.3 run empam –install mounter and follow the instructions on screen
2.4 Done! You can find mounter in /lib/mounter .

Documentation
SpoilerDO NOT LOAD MOUNTER WITH OS.LOADAPI!!!
Instead, do shell.run("/lib/mounter").

mounter.chroot(directory)
chroot's your PC to directory.
mounter.unChroot()
resets your root directory.
mounter.mount(source, target)
mounts source to target.
mounter.unmount(source)
umounts source.