I backup my stuff to an external hard drive. Between backups, that external drive is disconnected from everything. No USB connection to my computer, no power connection to my homes' electrical system.

This strategy of protecting your backups only works if you are diligent about remembering to do them (it's all manual). I am. Most people are not. On my wife's computer, I set it up for very frequent automated backups to an external hard drive, and her external is left connected to the computer, plugged in, and powered on. She never "does a backup", they are done for her automatically.

Depending on they type of person you are, your technical background, etc., you need to decide which is the best strategy for you. Personally, I do not do cloud-based backups because that means I'm trusting the provider to encrypt and make bandwidth available as good as they advertise. You can do your own encrypting before uploading to the cloud to add security, but then you're back in manual mode and might as well just backup to your own external drive. You don't get much space for free from the cloud-based folks, so you'll eventually have to start paying. Might as well spend that money on something you own yourself, like a second or third external hard drive.

A good USB3.0 thumbdrive is faster than the fastest of external hard drives (even USB3.0 hard drives). However, not all thumbdrives are created equal. Saying a thumbdrive is "USB3.0" tells you nothing about its speed. It only tells you that it supports the USB3.0 interface. I have seen plenty of USB3.0 thumbdrives that are barely any faster than a USB2.0 thumbdrive. So research thumbdrive speed first, and don't just buy the cheapest bulk pack of them at Costco because the label says USB3.0. That is meaningless (not totally meaningless, because a USB2.0 thumbdrive has an upper limit of how fast it can be, just due to the interface). So don't buy a USB2.0 thumbdrive these days. If your computer only supports USB2.0, you can buy an add-in USB3.0 card for under $15. I'd recommend doing that if you're currently stuck with USB2.0 on your computer. And research carefully which USB3.0 thumbdrive you buy.

My current recommendation, which is subject to change daily as new models come out, is the SanDisk "Extreme USB 3.0" thumbdrive. Larger capacity ones are marginally faster than smaller capacity ones. If you're buying a thumbdrive based on name, you have to be very exact in what you're looking for. For example, a SanDisk "Ultra USB 3.0" is a totally different beast than a SanDisk "Extreme USB 3.0". And next month, they'll no doubt come out with a different model named "The Ultra Extreme". Newer models are not necessarily faster then older models. Sometimes they are much slower, if the impetus for the new model was to hit a cost point and not a performance point.


An ancient A-Data USB2.0 thumbdrive:
Davids-Linux-Desktop ~ # hdparm -t /dev/sdd1
/dev/sdd1:
Timing buffered disk reads: 34 MB in 3.08 seconds = 11.02 MB/sec
Davids-Linux-Desktop ~ # umount /dev/sdd1


An old Optima Attache USB2.0 thumbdrive:
Davids-Linux-Desktop ~ # hdparm -t /dev/sdd1
/dev/sdd1:
Timing buffered disk reads: 54 MB in 3.02 seconds = 17.88 MB/sec
Davids-Linux-Desktop ~ # umount /dev/sdd1


A relatively new SanDisk Cruzer Glide USB2.0 thumbdrive:
Davids-Linux-Desktop ~ # hdparm -t /dev/sdd1
/dev/sdd1:
Timing buffered disk reads: 34 MB in 3.04 seconds = 11.20 MB/sec
Davids-Linux-Desktop ~ # umount /dev/sdd1


A "Costco special" USB3.0 Lexar two-pack thumbdrive:
Davids-Linux-Desktop ~ # hdparm -t /dev/sdd1
/dev/sdd1:
Timing buffered disk reads: 276 MB in 3.01 seconds = 91.73 MB/sec
Davids-Linux-Desktop ~ # umount /dev/sdd1


A SanDisk Extreme USB3.0 thumbdrive:
Davids-Linux-Desktop ~ # hdparm -t /dev/sdd1
/dev/sdd1:
Timing buffered disk reads: 502 MB in 3.01 seconds = 166.74 MB/sec
Davids-Linux-Desktop ~ # umount /dev/sdd1


A Western Digital Passport USB3.0 external hard drive:
Davids-Linux-Desktop ~ # hdparm -t /dev/sdd1
/dev/sdd1:
Timing buffered disk reads: 288 MB in 3.01 seconds = 95.64 MB/sec
Davids-Linux-Desktop ~ # umount /dev/sdd1


For reference, here is an internal SATA-3 SSD drive, but it's currently plugged into an old SATA-2 interface, so that severely limits its speed ... It would be much faster when connected to a SATA-3 interface:
Davids-Linux-Desktop ~ # hdparm -t /dev/sdc1
/dev/sdc1:
Timing buffered disk reads: 798 MB in 3.01 seconds = 265.49 MB/sec
Davids-Linux-Desktop ~ #
Connected to a SATA-3 interface, this SSD would be expected to come in around 490 MB/sec


Additional reference, an internal Western Digital Black SATA-3 hard drive, also connected to a slower SATA-2 interface, however that doesn't matter because a hard disk is not fast enough to outrun even a SATA-2 interface. SATA-3 gains you nothing in this case.
Davids-Linux-Desktop ~ # hdparm -t /dev/sdb4
/dev/sdb4:
Timing buffered disk reads: 398 MB in 3.01 seconds = 132.12 MB/sec
Davids-Linux-Desktop ~ #


Note that my SanDisk Extreme USB3.0 thumbdrive is FASTER than my both my internal hard disk and my external one. That's the state of things now - a good USB3.0 thumbdrive will be faster than your internal hard drive, but not faster than your internal SSD drive.

Also note, these tests I ran are for sequential read speed. You rarely do that on a computer (almost never). You do random access reads (which are significantly slower than sequential access reads, both for an SSD and a hard drive). Random access reads from an SSD run circles around random access reads from a hard disk, but these random access reads from an SSD rarely saturate the speed of even the older SATA-2 interface. So despite what the numbers above would appear to show, using a SATA-3 SSD on an older SATA-2 interface makes very little real world difference. It feels just about as fast as a SATA-3 interface (that's because normal computer operations are more random access and not sequential access).

Also note that most thumbdrives have a read speed that is faster then their write speed. On a good thumbdrive the two speeds are close. On a cheap one, the write speeds lag way behind. This is another reason to research your thumbdrives before buying. You want one with good read AND write speeds.

All of the above were plugged into a USB3.0 port on the computer for their test.