Many of my students keep asking me, how can I make a plug and play device like a pendrive, well in defiantly not very easy, I will give you two easy ways to do the same, it a easiest is… using a Easyusb module by daniel brodani..documentation is available on sparkfun..it a very simple module but the storage is limited to 2 MB, but its quite ok for a dataloger! .txt files are usually less than a KB. The interfacing is quite simple, it has UART lines TX and RX, connected them to your controller, go through the data sheet you will find commands to make a new file and write data, once the data is written u will see the file when u plug it in.
The second not so easy method to make a SD/MMC card data logger, to write the code on your own you need to have a clear understanding of SPI communication and FAT32 by windows, i was amazed initially to find out memory cards use SPI protocol! for all the newbies out there the is and AVR fat library which you can use! They also give sample code downloads for AVR, PIC and 8051. If you are comfortable working with Arduino, there is also a Arduino FAT libraryhere which is much simpler to use…and for all the lazy geeks there sparkfun had made a nifty micro SD card logger OpenLog using the arduino FAT library and its only 23$..so add USB compatibility to your projects now!..happy datalogging!!