It seems that a lot of Windows 7 users and having bizarre issues with the memory card support. According to many users, Windows 7 makes several permanent changes to the memory card which stops it from being used with other devices.
The modifications prevent you Windows Phone handset manufacturer Samsung warns that once you insert your memory card into the phone, "it will no longer be readable or writable on any other devices such as computers, cameras, printers, and so on.” Even AT&T has warned their users to only use microSD cards which are "Certified for Windows Phone 7" the issue with this warning is that no such cards are available yet.
But why are the cards not available you ask? According to Microsoft documents, certification is more than "a simple matter of judging its speed class" and includes many other factors, such as the number of random read/write operations per second, play a role in determining how well an SD card performs with Windows Phone 7 devices."
We hope Microsoft fixes these issues quickly, because after the initial hype and sales buzz wear off, it may develop a poor public image of Microsoft’s new range of smartphones.