How to enable SD MSC mount in Jelly Bean?
Seems as if Google is evil¹ and removed that option from Jelly Bean to enforce their DRM schemes². I found however a tool which restores it: Automated Framework Kit

¹Like we didn’t already know that.

²I have no idea what was the rationale behind it, but remember it’s the company which has a mail service that’s immensely popular despite not properly working with any major mail client.