There are  3 different types of  XD memory cards avaliable to date: Type H, M, and S and these cards are not all interchangeable in all cameras or readers! Check your owners manual or manufacturer's web site for compatibility with your camera or reader.

Below is a brief explanation of each card type:

Type H is the fastest with H standing for high speed with a write speed of 4MB/s and read speed of 5MB/s Olympus type H cards also allow you to do some special effects with the  Olympus picture editing  software, Type H cards are also the best for video recording. Type M has the highest capacity but is slower then type H with M standing for multi Cell technology. Type S is the orginal XD card with S standing for standard, Type S card  sometimes not marked with a type letter like the H and M cards. so if you see a XD card with no type letter it is a type S.