I am in the same boat as you. I updated to El Capitan and my Mbox USB interface is no longer recognized.
Mbox 2 USB Family 9.0.2.65 Driver (Windows) 0 MB Mbox 2 Driver 9.0 (Mac) 0 MB Mbox 2 Micro Driver for Pro Tools 9.0 (Mac) 0 MB Mbox 2 Mini Driver 9.0 (Mac) 0 MB. Avid Mbox Mini Driver is the free and cross-platform installer which provides the drivers needed for using Mbox Mini with Pro Tools 9 (or later), Pro Tools LE 8.0.5, or third party applications. The third generation Mbox Family products (Mbox Mini, Mbox, Mbox Pro) were announced on.
If you scan the forums you will find that other MIDI interfaces appear to have stopped working as well, it is not just the Avid MBOX 2. This points to the issue being on Apple's side, but I can't say for sure.
Linux (Debian-based) Install Instructions # NOTE: These instructions only work for 64 bit Debian-based# Linux distributions such as Ubuntu, Mint etc.# 1. Install our official public software signing keywget -O- sudo apt-key add -# 2. Add our repository to your list of repositoriesecho 'deb arch=amd64 xenial main' sudo tee -a /etc/apt/sources.list.d/signal-xenial.list# 3. Update your package database and install signalsudo apt update && sudo apt install signal-desktop. 
I am hoping for a fix from Apple in the next point release as I am mid-project as well..
Mbox 2 Driver Download
In the meantime I would recommend submitting a bug report to Apple. Hopefully the more of these they get the more likely we will see a fix.
Mbox Mini Driver Windows 10
Oct 13, 2015 7:04 AM