Question: Last year, the Western Digital external hard drive for my Windows 8.1 PC stopped working. I bought another Western Digital external drive, but it wouldn’t work, either.

I then bought a Seagate external drive and began using it last November. But by late July, I got an error message saying the external drive couldn’t be detected, even though I can see it listed in Windows Explorer.

I followed Seagate’s instructions to look at the hard drive’s properties and discovered that NTFS (New Technology Files System, a type of disk formatting) was missing from the list.

In addition, I’ve twice received warnings that Windows Defender (the antivirus software that comes with Windows 8.1) was turned off. I didn’t turn it off, so I turned it back on. What’s wrong? — L.D., Harahan, La.

Answer: If three different external drives have failed to operate properly, your PC must be the source of the problem.

First, let’s eliminate malware as a possible cause. Because your Windows Defender was turned off, run the free version of the Malwarebytes security program to make sure your PC doesn’t have any malicious software (see tinyurl.com/y3ju6oov).

Note that if you are using another antivirus program, Defender will automatically turn off. This is because you should never run more than one antivirus program; running two could dramatically slow the PC.

Now that your PC is safe, here are some potential fixes for the external hard drive problem:

Sometimes a PC’s USB ports can fail. Plug the external drive into a different USB port on the PC (you should have more than one). Or, if you are plugging the external drive into an external USB hub, try plugging it directly into a PC USB port instead.Open the PC’s “device manager (see tinyurl.com/y6kmu2q6 ) and let Windows automatically update the software driver for the external hard drive (see tinyurl.com/yy62zdnhand and scroll down to “Method 2. Update or reinstall drivers.”)If you have disconnected an external hard drive without first safely ejecting it, the drive’s software might have been damaged, preventing the PC from recognizing it. This seems likely, because the properties list for your Seagate external drive didn’t show “NTFS.” Seagate says this means the drive’s file system is corrupted and the drive needs to be reformatted.

Because reformatting will erase the drive’s data, you should try to retrieve any data on the drive first. You might be able to do that by assigning the drive a new letter so the PC can recognize it. (See tinyurl.com/yy62zdnh and scroll down to “Method 1. Assign a drive letter”.)

In the future, you can avoid damage to an external hard drive by safely ejecting it from the PC after use (see tinyurl.com/yazv9o9s and scroll down to “How to safely remove hardware in Windows 8.1.”)

Question: Why does my Windows 7 PC get updates? Didn’t Microsoft quit providing Windows 7 security updates in January? — M.Z., West Bend, Wis.

Answer: You’re receiving updates for Microsoft Security Essentials, an antivirus program. But no antivirus program will protect you against the big threat — newly discovered security flaws in Windows 7. You should move to Windows 10.