Hard drive that has the ability to "read" old data while it performs a write?
I wouldn't expect that such a feature could be implemented in a fashion that would be more efficient than simply alternating read and write commands.  If the drive has efficient buffering, the drive would likely respond to the first "read-sector" request by reading into a buffer the entire track containing that sector (the request would be reported as successful as soon as the requested sector was read, but the balance of the track would be available to honor future requests).  The drive would buffer the write request and report it as having completed immediately.  When the drive receives the request to read the second sector, the drive would either have finished reading the entire track or be in the process of doing so.  It would then buffer the second write request and report that as completed.  Depending upon the exact way the drive's buffering works, and the timing interactions between the drive and the host, the drive may or may not achieve the highest theoretically possible speed, but it should probably come pretty close.  I don't see how using a "read then write" command would be better than using separate "read" and "write" commands, since the drive would not be able to physically write a sector to disk until significantly after it was read.