score:2
A lot of threads on both Tripadvisor and Flyertalk confirms there is no problem. Even a less than a year old thread on 10-12 hard drives have lots of reports of no problems.
Photographer here who often does similar/the same: you're totally fine.
I have had TSA ask me to power up my laptop and tablet but never a portable HD
I've traveled with 3-4, no problem.
Upvote:0
As a developer traveling from Australia to Europe (working through my wife's bucket list) every year, I typically carry two laptops, 2-3 external drives and multiple mobile devices.
I have been asked to power up the devices on the way to the UK at a middle-Eastern hub because it's supposedly UK rules. Nobody asked me to verify the drives.
Never been asked to prove stuff works on any other trips to USA or other European destinations but that may be just changing rules.
Upvote:2
Adding to the answer of @chx (+1), I have traveled within Europe as recently as January 2016 with 2-3 external hard drives (and some other electronics) in my carry-on and while it did win me a selection for the explosives test, there was no problem whatsoever and security staff was very chill about it.