I just discovered that the most recent SD card standard (version 6 from early last year) includes an available "Low Voltage Signaling" (LVS) option that SD Cards can support. An overview is here, but briefly, without LVS if you wanted to communicate with UHS cards at 1.8V you had to first negotiate that option at 3.3V, then drop to 1.8V. With LVS cards hosts can bypass that 3.3V step and start at UHS-1 mode at 1.8V. My layman's reading of a few sources seems to indicate that the card is doing all of the work in detecting what the host can do.
...so can anybody tell me if that means (assuming I have such a card), I should be able to simply hook up the appropriate pins up to from the Edison (e.g., via the mini-breakout) to a SD card holder and be able to communicate with it? Any other components required (or recommended)? i'd be very pleased to not have to deal with lever shifters in this area for my project.
Hopeful I work something out, I went ahead and ordered an LVS card and will report back if what I find if no one else has an answer and there's interest (not much activity around here these days. :-)) Tip for others looking to try this: I wasn't able to find ANY cards advertising LVS compatibility, but if a card is an A2 class card, it is required to support LVS per the spec. Sandisk recently released refreshes of their Extreme MicroSD cards, at reasonable prices, bearing A2 class compatibility (and those are the only A2 cards I can find). Don't get the older A1 Extreme cards.