Dreamcast VMUs can store 128k in 200 "blocks". Third party memory cards increased the storage by having multiple virtual cards, each with 200 blocks.
With some hacking it is possible to free up some extra blocks on most memory cards, but there are reports that some games don't work with cards modified in that way.
Looking at the format of the VMU filesystem, it appears that there is support for more than 200 blocks. The number of blocks is stored as a 16 bit value, suggesting that at least 32767 blocks is possible. There may be other limitations, or compatibility issues as seen when modifying a card to unlock extra blocks.
Have any memory cards ever attempted to support more than 200 blocks without the virtual card trick?