After reading about the Everdrive-MD in this thread, I decided to get one. It came in yesterday, so I've been trying stuff out on it.
We've seen the pics on KRIKzz's web page (which is down right now), so I won't bother with photos on the cart itself. Let's start with a picture of how you chop up an old case for it.
Use a drill to make a hole through which you can use something to press the Master System button (it's not long enough to stick out through a standard case). Use a file to make a slot for the SD card.
After that, it will look like this when plugged into your MD.
Switch it on and the menu pops up. Note this is v3 of the Everdrive-MD OS on v2 hardware.
The way the Everdrive-MD works is interesting - it has 8MBytes of flash on it. The OS runs from the first 4MB. You select a game to copy from the SD card into the second 4MBytes of flash. Choose "select new game" in the menu to go to the SD card browser. There you can select a game to burn to flash. Note that SD cards must be FAT16 formatted. FAT32 formatted SD cards will show an error when you try to use them. It DOES support long file names, so that's a good thing. It does NOT sort the file names, which is a bad thing. You'll be hunting through pages of files looking for the game you want (and Murphy's Law states it will ALWAYS be on the last page you check). After it is burned to flash, you can play it from the top selection of the main menu.
It takes about 20 seconds to erase 4MBytes of flash... less for smaller games. Then the game is read from the SD card and burned. This is rather fast, taking only 24 seconds for 4MBytes. The combined erase plus burn time of just over 43 seconds is almost half the 79 seconds it takes the Neo Myth MD to load a 4MByte file off SD card into PSRAM. At the moment, the speed of the Everdrive-MD is offset by the time it takes to locate the game. I look forward to a revision of the menu that sorts the file names.
MD games ran great on my CDX - no troubles on anything I've tried. When his web site says it doesn't work with 32X games, what it means is it doesn't work with the 32X. While the menu starts, it won't properly erase the flash, nor does it burn the game to flash. Even MD games don't work with the Everdrive-MD through the 32X. You HAVE to run this directly on your MD. Also, while SMS games are supported, I was unable to get any to run on my CDX. They DID work fine on my Nomad, so this is again probably a hardware issue of the Everdrive-MD REALLY wanting to work on just a MD. Use anything else and it may or may not work.
So what are you getting for your money? A flash cart that handles nearly any MD or SMS game, but only on plain MD consoles or the Nomad. Any other Genesis model or compatible (it didn't work at all on either the RetroGen or the GenMobile) probably won't work with it. However, if what you have is a MD or Nomad and all you want it to run games from an SD card, this is probably the cheapest way to do so.
EDIT: v4 of the OS is out and fixes the issue with SMS games on the CDX. SMS games work fine now. Also, you can burn a 32X game to the flash on a MD/CDX/Nomad, and then play it on a 32X. That works fine. The issue with the 32X is being looked into.