Here's one for -rc...  either this bit is assumed on most modern drives,
or nobody uses the 6-byte commands for read/write (which is very
likely).  Either way it definitely wants fixing.

Since we use the modern "LBA" method of addressing sectors, this bit
must be present in the Device register when issuing a read or write. 
There are three code paths that need this bit, and only two of them
actually set it.  This patch fixes the third (and apparently never used)
code path.