Is it possible to write a filesystem for Windows in pure usermode, or more specifically purely in managed code? I am thinking of something very similar to GMAILFS. Excluding what it is doing under the covers (GMAIL, Amazon, etc..) the main goal would be to provide a drive letter and support all of the basic file operations, and possibly even adding my own structures for storing metadata, etc..
It's difficult. I'd take a look at some projects which have done some of the hard work for you, e.g. Dokan.