7 things you should never connect to your PC's USB ports

Your computer's USB ports can be confusing. Some are color-coded ; some are super fast; some are super slow. But beyond all that, they're pretty flexible: if the cable fits, you've got yourself a connection.

Does that mean everything you can plug into a USB port belongs there, though? Nope! Here's what I never plug into my desktop USB ports (and you shouldn't either).

A device with a wet, corroded, or damaged connector

Don't risk your expensive PC

I guess this is a given, but this is the worst thing that comes to my mind as I sit down to work on this article. If you have something that's in terrible condition -- something where the port is literally corroded or wet -- please, don't even try. At least not on your main PC.

A USB connector packs power and data contacts into a tiny space. If the plug is wet, corroded, bent, etc., those contacts can connect in ways they were never supposed to. Best case? It won't work. Worst case? It could cause a short, trigger the PC's overcurrent protection, or damage the port itself. All bad outcomes, if you ask me.

Visible damage is also a good reason to stop experimenting, by the way. If the device is shady to begin with, don't move it from one port to another in hopes of it working. Replace cables with exposed wiring, melted plastic, heavy corrosion, or bent contacts.

A phone or an accessory with a swollen battery

USB charging isn't going to make anything better here...

You may have seen a phone or another type of device that appeared weirdly swollen. That's due to the battery. If it's already swollen, it's good for nothing, and you may notice the symptoms through more than just a dented back. The screen lifting away from the phone, the casing separating, or it just looking rounded ... that's bad news.

Plugging this into your computer will try to feed energy into a dying/dead battery, even if the USB port charges it more slowly than a wall adapter (you generally shouldn't use your laptop to charge up a phone ).

But doing this is risky, not for your PC, but for the device itself. You're creating a risk of further swelling, overheating, leaking, or, in ultra-extreme cases, fire. Disconnect the device and stop using it rather than plugging it through a gentler, slower source of power.

A passive USB-A-to-USB-A cable between two computers

For kind of obvious reasons

If you have two PCs that you want to connect, a USB-A cable seems like a logical solution. However, the rectangular USB-A ports on two computers are normally both host connections. They're designed to control attached devices and supply them with power, not to be connected directly to another host. So, a basic USB-A plugs at both ends doesn't contain the electronics needed to negotiate a proper PC-to-PC connection.

A badly made passive cable may also connect the two PCs' powered USB lines together, which can cause unpredictable electrical problems. If you want to connect your two PCs, use your local network, an external drive, or a purpose-built USB transfer cable.

USB-C or Thunderbolt cables may support communication between computers, so you're in the clear there.

A device that already caused an overcurrent warning

Once bitten, twice shy

An overcurrent warning essentially means the device tried to draw more power than the USB port could safely provide, or the computer detected something resembling a short circuit. I probably don't have to tell you that nobody wants anything with the words "short circuit" anywhere near their expensive PC.

This could be caused by a damaged cable, faulty accessory, overloaded unpowered hub, or a device that requires its own power adapter. No matter the reason, the warning shouldn't just be clicked away and forgotten about.

If you get this warning, unplug the device and investigate before you connect it to your PC again. Try a known-good cable, check its documented power requirements, and confirm that any required external adapter is working. Don't keep moving the device from port to port; I'd honestly just keep it away from my PC, period.

A cable or adapter from an unknown source

A normal-looking cable can hide active hardware

Testing your USB cables will tell you that these contraptions aren't just passive collections of copper wires: some contain controllers and other electronics inside their connectors. That's helpful if you know what's going on inside, but it can be something malicious, which, obviously, you want to shield your PC from when possible.

Deliberately malicious cables can imitate keyboards, network adapters, and other things you'd normally trust. Once connected, they may be able to type commands or redirect traffic without asking you to open a suspicious file first.

The odds that a random cable is an advanced espionage gadget are, admittedly, minuscule. But do you really want to take that risk? Besides, it's not all malware all the way down -- they can also be badly wired, damaged, or unable to handle the power they claim to support. All in all, bad news.

A random USB drive you found somewhere

Please don't do this

Similarly to the above, plugging in a mystery flash drive to find out what's stored on it is risky. If you found it in your drawer, sure, I guess (unless there's a risk it's not yours to begin with). But if you found it somewhere and it's not your drive, it could be infected with something or contain malicious executables. Trust me, you and your PC don't want any part of that mess.

A USB killer

Pretty self-explanatory

A USB killer is exactly what it says on the tin. It's built to damage electronics, and it draws power from the USB port. It uses that energy to charge internal components, and then discharges a much higher voltage back through connections intended for ordinary USB communication. Depending on the protection built into the PC, it may destroy the port itself, the USB controller, or even the motherboard if it's your unlucky day.

There's virtually no reason why you'd connect this to your PC, so please don't treat it as a "Mentos in a bottle of Coca-Cola" type of experiment. Leave it well enough alone.

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Choose caution when dealing with your USB ports

Most USB devices and cables are super safe, and your USB ports can do a lot of useful stuff . But don't treat them as testing grounds for just about anything that comes to mind. If it's broken, unknown, or unsafe, don't bother testing it on a computer you care about.