The Yun features a SD card socket. Do you need to have the OS on the SD Card to use the Yun (like RaspPi)? Or is there a flash chip on board to hold the OS?
The customized version of the OpenWRT os, called Linino is pre-loaded on a 16 MB of flash memory directly connected to the Linux processor.
You can expand the storage memory adding a microSD card.
taken from the product page:
http://arduino.cc/en/Main/ArduinoBoardYun:
MemoryThe ATmega32u4 has 32 KB (with 4 KB used for the bootloader). It also has 2.5 KB of SRAM and 1 KB of EEPROM (which can be read and written with the EEPROM library).
The memory on the AR9331 is not embedded inside the processor. The RAM and the storage memory are externally connected. The Yún has 64 MB of DDR2 RAM and 16 MB of flash memory. The flash memory is preloaded in factory with a Linux distribution based on OpenWRT called Linino. You can change the content of the factory image, such as when you install a program or when you change a configuration file. You can return to the factory configuration by pressing the "WLAN RST" button for 30 seconds.