I just noticed this today, I was doing some troubleshooting for a client's PC and had to use an external enclosure (3.5" with its own power supply) in my troubleshooting.
What I've noticed is if I plug in any external harddrive that is 3.5" with its own power supply the fans in my MBP jump up to 6000rpm.
If I use my mini-external HD enclosure (takes a laptop HD instead of a full size HD) using both USB connectors so there's no external power, its pulling power off of USB... I have no fan spin up issues.
As soon as I unplug the ext HD within 30 seconds the fans are quiet and back to normal speeds.
And if it helps to figure this out, my CPU and Memory temperatures are over 60 C (140F). Way above my normal running temps.
Any ideas on why this is happening? Is it a normal thing? (this is my first MBP, I didn't experience these issues on my old laptop (ibook). Or is it something I should be going to Apple about to have repaired?
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