The standard PlayStation 2 memory card has an 8 MB capacity and uses Sony's MagicGate encryption. This requirement prevented the production of memory cards by third parties who did not purchase a license for the MagicGate encryption. Memory cards without encryption can be used to store PlayStation game saves, but PlayStation games would be unable to read from or write to the card – such a card could only be used as a backup. There are a variety of non-Sony manufactured memory cards available for the PlayStation 2, allowing for a larger memory capacity than the standard 8 MB. However their use is unsupported and compatibility is not guaranteed.

Source: Wikipedia, "PlayStation 2", available under the CC-BY-SA License.