The xD Picture Card was developed by Olympus and Fujifilm and released in mid 2002. The xD card then had improvements made and labeled Type M and Type H. The improvements of the xD flash memory technology where done to increase storage capacity and/or speed performance.
The original xD format held between 16MBs and 512MBs.
The following version was Type M. This increased storage capacity to 8GBs and released in the beginning of 2005.
The next format version, Type H, increased performance of data transfer and has a storage range of 256MBs to 2GBs. Type H xD card was released in late 2005.
Since Type H has better data transfer rates, this type of card is needed if using a camera for capturing video. Type M is too slow.
Type M uses a flash memory typology called MLC or Multi Level Cells. This approach increases storage capacity but at the expense of performance. For example, if you use a Digital SLR camera or looking to capture video, then Type M xD memory would be too slow.
Here are some data transfer rates:
Bottom line – most xD Picture Card formats are compatible with each other, here is the most current incompatibility list and it’s worth noting that some card readers do not support the newest format…although it means you probably have a really old card reader (just get a new flash memory card reader for $15).