Nearly 20 years ago I wrote DOS For Dummies, the biggest, bestest-selling DOS book ever. In the back of the book, in the Part of Tens, I listed ten things you should never do on a DOS computer. Here they are with my up-to-date commentary on the relevance of my ancient admonitions.

Don’t Switch Diskettes

Way back when, a lot of PCs were sold without hard drives. Even when they had a hard drive, most DOS users preferred using floppy disks. Similar to today’s removable media cards and flash drives, it’s possible to yank a floppy out of a drive when it has open files. What would happen in the old days is that people would switch diskettes thinking that they could re-save a file on a new disk. Nope. The end result was that both disks had their information corrupted.

Don’t Take a Disk Out of the Drive with the Light On

Seriously, people would do this: Yank a disk out of the drive when the light was on, which meant that the disk was being accessed. The end result: A corrupted disk.

Don’t Turn Off the Computer When the Hard Drive Light is On

Remember, early PCs had an on-off switch, not a power button. In fact, one of the most frustrating things for a DOS user was knowing when it was “safe” to turn off the computer. One thing was certain: You didn’t turn off a computer when the disk drive light was on.

Don’t Reset to Leave an Application

One of the worst offenses made by early DOS users was rapidly turning the on-off switch off and on to “quit” an application. It worked, of course, but it also corrupted the hard drive and led to other problems.

I mean, seriously: It was spread by word-of-mouth: To quit a program on the computer, you turn the thing off and on really fast.

Don’t Plug Anything Into the Computer While It’s On

And you thought USB was stupid? Back in the old days, you never, never connected anything to a PC when the power was on. Not even a mouse. When you did, you fried either the device or the PC itself.

This admonition makes you kind of appreciate our USB era, where you can attach and detach items to a computer all day long without turning it off and then on again.

Don’t Force a Diskette into the Drive

It may seem like a cliche, but back in the early days people did wedge floppy disks into just about any crack they would fit into on the PC, especially between the drives. The same people who were fearful about losing a file and timid about following directions would think nothing about fist-pounding a floppy disk into an air vent.

Never Format a High Capacity Diskette to a Low Capacity

And

Never Format a Low Capacity Diskette to a High Capacity

Yeah, back when the computer was too stupid to recognize disk capacity through the drive hardware. You had to type a special command to format a high capacity drive at high capacity. You could use the same command on a low-capacity disk, and DOS would report that the operation was successful, but you couldn’t store anything on the disk.

Never Load Software From an Alien Diskette

This was an early warning against malware. Here’s my original warning text:

Only buy software “shrink wrapped” from a reputable computer dealer. Any other program you get, especially those on cheaply©labeled diskettes, is suspect. Don’t trust it! This is how computer viruses are spread, so it’s best not to load anything from an alien diskette. And, for God’s sake, never boot from such a diskette.

Without the Internet, floppy disks were the number one way that viruses spread between PCs. It was a successful delivery method. Lots of idiots suffered.

Never Use These DOS Commands

Why on earth would Microsoft introduce and make available — without warning — commands such as CTTY or RECOVER?

The CTTY command redirected standard input to the COM port on the PC, which was effective only when you added (and properly configured) an external terminal for DOS. Otherwise the command unhooked the console (screen and keyboard) and you were left with a hung computer. Nice.

The RECOVER command had a nice name, but its effects were deleterious. It assumed, without any prompt, that your hard drive was damaged and needed a bit-level recovery. So that’s what the program did, chomping through your entire directory structure replacing valid files with useless junk. It was a nasty, nasty command and — get this — it survived for years as a DOS “tool.”

In fact, RECOVER is still a command line tool in Windows today. Don’t use it. It’s misnamed and should have been killed off decades ago. So, you see: Even though it’s old advice, some of my 20-year-old DOS admonitions are still worthy!