The Dreamcast is emulated by the Libretro Flycast core, available on every machine. Libretro Flycast Next is also there on PC, Raspberry Pi 3, 4, 5 and Pi Zero 2, and Reicast on Raspberry Pi 3 and Pi Zero 2. Your games go in /recalbox/share/roms/dreamcast.
Accepted formats are .gdi, .cdi, .chd, .cue/.bin, .iso, .elf and .m3u. The light gun and the keyboard are optionally supported.
That is the question on this system, and the answer is not the obvious one.
Redump images (.bin/.cue) are built for preservation: they keep data added during industrial pressing, which emulators never read and have no use for. TOSEC images (.gdi) reproduce the tracks and data exactly as the console's GD-ROM drive read them — which is precisely what the emulator expects.
For playing, prefer .gdi.
.chd?The .chd format (v4 and v5) was designed to compress .gdi images. Converting a .gdi to .chd is therefore entirely recommended: you save a lot of space and lose nothing.
Converting a Redump .bin/.cue to .chd, on the other hand, is asking for trouble: the emulator will not know where the set came from and will expect tracks aligned the TOSEC way. Realigning them means going through third-party software, stripping data and rebuilding a .gdi — far more work than simply getting a TOSEC image in the first place.
Neither format is "better" in the absolute: Redump is excellent for archiving, TOSEC suits emulation. Choose according to your use.
See also Which romsets to use with Recalbox.
A handful of Dreamcast games span several discs: Resident Evil Code: Veronica, Skies of Arcadia, Grandia II…
At the end of the first disc, the game offers to save. Go back to the menu with HOTKEY, launch the second disc and load that save: the session picks up where it left off.
You can also group the discs into a single entry with an .m3u file, and swap discs without leaving the game.
To erase or manage the content of your virtual memory cards, you need to get to the console's own menu. Two ways.
HOTKEY + left stick up./recalbox/share/roms/dreamcast, renamed with the .cdi extension — Dreamcast BIOS.cdi, for instance.