Secure Digital (SD) is a flash memory (non-volatile) memory card format used
in portable devices, including digital cameras, handheld computers, PDAs and
GPS units.  SD cards are based on the older MultiMediaCard (MMC) format.

==> SIO2SD, by Jakub Kruszona-Zawadzki

http://sio2sd.gucio.pl/

SIO2SD is a device which makes it possible to load games/applications into
8-bit Atari computers via SIO interface from SD/MMC cards.

Device abilities:
 - Works with SD/MMC (FAT12, FAT16 and FAT32 formats)
 - Handles ATR (rw), XFD (ro) and COM/XEX (ro) file types
 - 16x2 LCD display allows to "walk" catalog tree and choose files to load
 - Handles SIO with turbo (speed 51200kbps - hsindex 10)
 - All densities with 128B and 256B sectors, including 16MB disks
 - Handles drives d1 to d4 (special version d1 to d8 available)


==> SDrive, by C.P.U. (Radek Sterba (Raster) & Robert Petruzela (Bob!k))

http://raster.infos.cz/atari/hw/sdrive/sdrive.htm

The SDrive is a device that connects to Atari XL/XE's serial (SIO) port and
simulates an Atari floppy disk drive with full read/write access to programs
and data stored on a Secure Digital (SD) flash mamory card.

Main features:
 - Supported flash cards: Secure Digital up to 2GB size, FAT16 filesystem
 - Maximum number of drives: 4 (D1: to D4:) + 1 special boot drive
 - Supported SIO transfer rates: 3.5 to 128kbps (standard 19 and 69kbps)
 - Supported disk images: ATR, XFD, size up to 16MB, 128 or 256B sectors
 - Supported executable files: COM, XEX, BIN.... (any filename extension).
 - Device controlled by software running on Atari from the SD card, which can
   be therefore easily updated/replaced
 - Drives swappable on the fly by buttons
 - Write protect/enable switch
 - SDrive ID number selection switch - simultaneous use of up to 4 SDrives
 - Low cost design - no LCD, a few LEDs, cheap DIL28 Atmega8 MCU, single sided
   PCB
 - Firmware and software source code freely available

Special features:
 - Buffered reads for speedup
 - Delayed writes for speedup and greatly reduced flash write cycles
 - Built-in bootloader requiring less than 256 bytes including sector buffer,
   relocatable in the $0500-$05F7 to $BE00-$BEF7 range, with SKCTL
   initialization before every block. Supports executable files of up to 8MB
   size.
 - Directory with filename simulated for active files in drives, data handled
   through standard 128B sectors. Executable files can be run from most DOSes
   or Q-MEG. Random data files with arbitrary suffix can be activated and
   opened by a program through DOS or copied to disk images. (Note: 80KB file
   size limit applies to standard DOSes, 8MB to Q-MEG and MyDOS)


==> SDrive NUXX, by Steve Vigneau / c0nsumer (based on SDrive by C.P.U.)

http://nuxx.net/wiki/SDrive_NUXX

Based on SDrive by C.P.U.  Changes from the original SDrive:

- A readily available enclosure and custom end panels with cutouts and
  artwork.
- An SIO connector footprint. This allows a standard Atari SIO
  connector to be used, allowing easy connectivity with any of the compatible
  Atari 8-bit computers. 
- Incorporates a low-cost AVR programmer allowing a SDrive builder who doesn't
  have AVR programming hardware readily available an easy method of loading
  the firmware on the microcontroller.
- The built-in Brown-Out Detector has been enabled with a 4.0V threshold.