Back in 2008 when the SD2IEC storage device came on the scene for the Commodore 64, it wasn't compatible with the .D64 disk image file format, instead it used what is refered to as the M2I format.  Here is a collection of games released by "znarF" that is in the old format.  Aparently based on this post over at the Nightfall Crew's web page, you need to convert these to P00 format to be used with newer SD2IEC firmware. 

For reference, this post on the Lemon64 forum is why I went hunting for this collection.

Update - You can go here to find the package in what I believe was it's original resting place and find a lot more information about M2I.