I want to know what items are allowed into the airplane. I know that, for example, laptops and tablets are allowed. However, what about power bank/batteries or external hard drives? My question is about the electronic devices I can bring into the airplane.
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All electronic items are allowed except those that are prohibited by the airline; or the country's security rules. These include:
Large batteries (see Can you put an ordinary Mac laptop (with the battery) in checked luggage? for some details on the capacities).
Anything that can be used as a weapon.
Electronic items that do not turn on (cell phones, laptops, tablets, e-readers, etc.) or are malfunctioning may not be allowed, depending on the airline and your itinerary.
Anything that cannot fit in the carry-on size and weight requirements.
Please carefully pack your electronics items, making sure all cords are wrapped and any DVDs, CDs, or cassettes are removed from their devices. While you can pack expensive, fragile electronics items in checked or carry-on bags, we recommend packing them in your carry-on. Laptop computers, full-size video game consoles, CPAP machines, full-size DVD players, and video cameras that use video cassettes must be removed from their carrying cases and submitted separately for x-ray screening. Small and portable electronic items (smart phones, tablets, portable games, etc.) do not need to be removed from their carrying cases or carry on bags.
For items you wish to carry-on, you should check with the airline to ensure that the item will fit in the overhead bin or underneath the seat of the airplane.
Even if an item is generally permitted, it may be subject to additional screening or not allowed through the checkpoint if it triggers an alarm during the screening process, appears to have been tampered with, or poses other security concerns. The final decision rests with TSA on whether to allow any items on the plane.
You are generally allowed to carry power banks and external hard drives with you as carry-on baggage. I had some AA batteries confiscated when I tried to take them with me at Frankfurt. They were for my camera but the officials did not agree. As for other electronic goods, please be more specific so we can let you know from our experiences. Also, this varies with respect to airports and countries as well.