The 8Tb WD SMR drives I have use a 40gb chunk of the surface for non-SMR reads and writes as a buffer. Altering anything in an SMR track requires copying the data from that track and tracks near it into the buffer, then laying them back down into the SMR region sequentially. Like erasing a flash block before writing. Unfortunately, this means that the drive's performance takes a dump from 200mb/s down to 300kb/s if you do enough transfers to fill the 40gb buffer, or attempt multiple transfers at once.