USB is way too complicated to implement in small AVRs.
See the very long thread about how it can be done: http://www.mcselec.com/index2.php?option=com_forum&Itemid=59&page=viewtopic&t=7537
My suggestion would be SD or microSD card and AVR-DOS.
SD card is gross overkill for the storage required, but the cards are cheap, and so are the connectors. You could do the same with a small eeprom, but it would have to be mounted in a holder of some sort, and a matching connector.
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