Hi Rich,
I am just reading this a second time and it's March 23rd 2019. I hope you were able to find a solution, but I found one and I'm happy with it. Avid did have an updated driver for my Mbox Mini, but it took a while to find it. It came out in 2018 I believe. I am using OS High Sierra.
My problem was that I hadn't used my mini in years. I thought it was an Mbox 3rd gen. When I looked at it closer I realized it was a mini so I searched for drivers on the avid website. At first I thought it was no longer supported so I downloaded similar model drivers and kept getting "not connected". Once I found and installed the correct ones I was good to go.....I must add that I find the avid website a bit confusing though and that's why I spent till 3am last night figuring this out.
Perhaps this info might help you or you may have already bought a new interface or that driver isn't available for your model. In any case I hope you figured it out by now. This was my link. Maybe yours is close by?
http://avid.force.com/pkb/articles/e...x-Mini-Drivers
Good luck!
Frank