This page is presently out of date. More recent work, with significantly higher speed and more functionality, is on sourceforge and in the Propeller object exchange.
News: I've released fsrw1.6 which fixes a bug when you mix pputc() and pwrite() in the same file.
Here's some code to support FAT16 filesystem reading and writing from the propeller, as well as some low-level Secure Digital block I/O code.
The current version (download here) is 1.6, last updated 23 September 2008.
If you try this code please send me email (rokicki at gmail.com) on how it works for you and what is lacking that you need.
More details on the code can be found here.