According to this post:
http://pcedev.blockos.org/viewtopic.php?f=5&t=98&start=10#p737

The 'read sector' routine should send 3 bits after completing a sector read (but your code doesn't do this yet). I have tested this, and it seems necessary.

In the same post, the 'write sector' routine has a slightly different end-sector sequence. It seems to read 2 bits, then send 3, then check the value of the last bit read. I haven't checked whether this is necessary though.