i wonder if i could save a file using arduino? well, we have this idea of a digital multimeter which is capable of saving the measured data and then transfer it through a USB port. can arduino be programmed to save such files? thanks 
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The Arduino can fairly easily store files on an SD card. The file system software is fairly large so you need a 328P at least to get anything done. Check out the built-in SD library in Arduino 1.0.
thanks a lot. if it's okay, can you explain it further? we really can't put together our ideas. we are in our senior year in college and having our thesis. we proposed a digital multitester which can save the measured data either as a bitmap file or text file then convert it to bitmap? (we don't know what's better--any sugguestion? ;)) then a built-in USB port will be included. files also can be transferred through Bluetooth.. .and we're planning to use arduino as the main processor of the device.. hmm. also, what LCD should we use? thanks! 
Everything you describe can be done and examples already exist. I would suggest starting with both a hardware and software block diagram and then investigating each block.
save the measured data either as a bitmap file or text file then convert it to bitmap? (we don't know what's better--any sugguestion?
Store your measurements in raw (=text) format, e.g. use space delimited files , creating bitmaps and graphs can always be done later on any computer.
Generating bitmaps on the Arduino can be done but takes time which reduces the time for measurements (e.g. freq).