The instructions for upgrading the firmware say you should only have the drive you want to flash plugged in. Unfortunately some people skipped that bit. A feature of the firmware updater app is that it will quite happily flash any drive that looks like its target. So if you happen to have more than one drive plugged in (the same model as the drive you want to flash) then that will get done too.

Meanwhile I'm just sitting tight and waiting for some new (and tested!) firmware to appear so that I can upgrade my 2 x 500GB which have the (alleged bad) SD15 firmware. But I'll make sue the forums are full of success stories before I attempt anything.