I've had a lot of questions about some details of what the Satiator will and won't do - so here is a full specification of what it is and isn't.
The Satiator is a plastic cartridge which slides into the Video CD port on the back of the Saturn. You can close the Video CD door to conceal it, or open it to access the SD card slot. The SD card slot is push-to-eject and takes a fullsize SD card. Capacities of at least 256GB will be supported and I expect to support larger cards when they are available. Cards must be FAT formatted.
When the console is powered on with no game in the CD drive, the Satiator takes control of the boot process. It loads a menu program off the SD card. The menu lists the playable disc images on the card; currently, it supports ISO and BIN/CUE. When you select an image, it loads and boots that image. Region restrictions can be bypassed by the menu (using the code I wrote to do the same thing in Pseudo Saturn). The menu can be updated just by replacing the menu.bin on the card. The menu is open source and so anyone can mod it if they wish.Homebrew software running on the Saturn can also access files on the Satiator.
Firmware of the Satiator itself can be updated via the SD card as well. I think this is unlikely to be needed once beta is complete.
The Satiator does not emulate a Video CD card, nor does it allow pass-through to a real Video CD card, so Video CD content cannot be played. The Satiator doesn't use the top cartridge port on the Saturn, nor does it emulate an Action Replay or backup cartridge.
There is a USB port on the Satiator but it is currently only used for production test.
There's a red/green LED for status and error info. If you mess up the card formatting or forget to put the menu.bin on the card then there will be a particular blink pattern to tell you what's up.
What did I miss? Who's got questions?
Edit #1: Things I Missed
Thanks to everyone who chimed in in the comments so far! Here are some more important things:
The top cartridge port retains its normal function. You can still use RAM carts, Action Replay, et cetera. I don't know yet how the AR boot sequence interacts with the Satiator's, will find out soon.
The Satiator doesn't currently hook into the native game saving infrastructure, but there's no reason that there couldn't be an applet as part of the menu that lets you transfer saves between SD and the console's internal RAM. I haven't written one at this point.
The loading times are identical to real hardware because it paces the sectors out the same as a real drive for compatibility reasons. Some titles will depend on the exact CD drive timing and the Saturn's CD Block itself can only handle a certain data rate, so while I may implement a turbo mode in the future, it will be a configurable option rather than on by default.
This also means that the choice of SD card is not critical - it only reads at a few hundred kB/s. As long as your card isn't actually faulty (fake capacity or bad sectors) then the Satiator is not picky.
The firmware should shortly support exFAT, but the firmware update mechanism will only ever support bare FAT due to code size constraints. I think it should be quite manageable to have 512GB+ FAT formatted cards, even if it's not exactly pretty.
The BIN/CUE support handles multiple files (eg .iso + .cdr audio). It doesn't currently handle WAV tracks but that should be easy to add. It will never handle MP3 tracks due to CPU constraints.
When it's released, which will be in 2019, patrons will have first dibs on ordering in some way. I'm not sure how exactly that's going to work because distribution isn't in place yet.
Anything else? :D