I've cloned my internal SSD of Sierra to an exact same SSD, and just once changed the start up to check all is OK with the clone. Started up fine, took longer because it's USB connected but other than a few things like 'Downloads' are in a completely different order to the internal one for some reason, seems OK. But, since I've checked it out and disconnected the external, the shut down and start up using internal is taking more than twice as long. The white screen before the Apple wasn't even there prior. It started up with the Apple logo straight away, Now it's a good minute before I see it, and as soon as it start up now the fans are on full pelt. Other than 3 x mdworker and kernel task in Activity Monitor, I don't see anything untoward. But why after using a different start-up disk and only once to check it out, has it changed the behaviour of the internal disk so drastically, Any idea?