os supporting sd card read and write

Environment based on Linux of course

Google Android • Samsung Bada • Mozilla Firefox OS • GNU/Linux • Apple iOS • Mer • NetBSD • OpenBSD • FreeBSD • RIM BlackBerry OS • Jolla Sailfish OS • Symbian OS • Tizen • Ubuntu Touch • Microsoft Windows (RT • Phone) • Alibaba Yun OS

Environment based on Linux of course

Google Android • Samsung Bada • Mozilla Firefox OS • GNU/Linux • Apple iOS • Mer • NetBSD • OpenBSD • FreeBSD • RIM BlackBerry OS • Jolla Sailfish OS • Symbian OS • Tizen • Ubuntu Touch • Microsoft Windows (RT • Phone) • Alibaba Yun OS

FreeRTOS would be another one.

I agree there is really a wide choice!

Guess ARM7 need some RTOS instead of rich OS such as Linux, try goole eCos, MQX, ucOS. And Good luck.