I did know that ostraca (or ostraka), small shards of broken pottery, were used during a certain period in Athens to vote (ostracism). What I did not know was that such shards were also used for tax receipts and other official documents as well as letters. They also had inscriptions meant to function as charms, curses or amulets. But most surprising to me was to find out that they were also used for note-taking. Makes sense, just did not know.
Papyrus would have been too expensive, wax tablets too formal and cumbersome. Pottery shards were available everywhere, just as as scraps of paper today. Size and flatness of shards were prized. Accordingly, those broken from large amphorae seem to have been favored. Sometimes, a number of shards were kept together in a wooden box. Scratching the letters was possible, but some kind of ink was used more often. Early ink would wash off too).
Some claim the practice started with the Egyptians.
Whatever their use in antiquity was, they are a far cry from a USB stick or a SD card.