Hi, an External Hard-Drive would do you well. I would recommend a USB Pen Drive, capacities these days go up to 256gb, they are simply plug and play and require no other source of power.
I think these things are way too unsafe to store large amounts of data you consider important.
Hard drives are still the de facto back-up standard because they are reliable, fast and if they are damaged, there are plenty of specialised companies out there that can retrieve data succesfully if they do become damaged.
Is portable as reliable as a desk top drive? Lots of questions........ Any one have any suggestions?
I prefer the 3.5" drives because they are faster and heavier. The reason they are powered by the mains is because they require 12v to operate, which the usb can't deliver.
The 2.5" can be powered by USB ports but you tend to get fewer GB per $.
I'd keep one copy on the computer, one copy on a hard drive in a safe at your home and one copy on cloud storage or another external location if you're going to make a photo-archive. If the data isn't all that important, I guess you could stick to only storing it on two locations.
Invest in diagnostic software such as Hard Disk Sentinel to continuously monitor drives...