fix reading of partial clusters in fat32

[deitch]

Fixes #108

[deitch]

@RafaelMarinheiro can you try the branch here and see if it resolves your issue #108? The tests pass now.

[RafaelMarinheiro]

Thanks! This fixes the issue with io.Copy, making it work as expected!

Having said that, the code with io.Copy is MUCH slower than the code with ReadAll and a single write. I guess this is happening because now you need to deal with FAT32 bookkeeping on every single Write call. You could have avoided this by postponing some of that bookkeeping to a Close() method in the file, but unfortunately the current API does not have anything like that in it.

Thanks again for the fix!

[RafaelMarinheiro]

This seems to have fixed #109 as well (at least now I can write a large number of things to the same directory and they show up afterwards). Could it be related in some way?

[deitch]

Possibly. The Read was broken, which might have done it. Let's merge this in, then follow through on #109