7Zip and winrar, et al, only encrypt a file or group of files, but not the whole volume (aka, "drive"). For whole volume encryption you'll need a program on your computer to do the encryption/decryption. Plug in the external drive, put your password into the decryption program, the external drive will get a drive letter, and you can read/write files just like any other drive.
That will be fine if you always use the external drive on the same computer, but keep in mind you'll need to install the same software on another computer if your primary computer fails or becomes corrupted for some reason. Don't take that requirement lightly. You'll want to be able to reinstall the same version of the software to maximize your chances of success when all else is going south.
With that in mind, your three main options are:
Windows Bitlocker:
Bitlocker is built in to certain Windows versions, but not all. If you have it and use it, you could be limiting your recovery options if you main computer dies. You'll have to find another computer with Bitlocker, and you'll have to hope Bitlocker versions stay backward compatible. (They should, but it depends on how much you trust Microsoft -- for instance, look at the mess MS has made with Windows Backup over the years, where backups created on one version of Windows can't always be read by another version of Windows.) Some people are also suspicious as to whether Microsoft has built a backdoor into Bitlocker for the government's benefit.
Drive manufacturer proprietary software:
If your external drive was made by Western Digital or Seagate, it's likely it came with a copy of an encryption program you can install on your computer. If it didn't, or if it did and you deleted it, you can probably download it from the WD or Seagate website.
Universal (third-party) software such as Veracrypt:
This would be my recommendation. Veracrypt is free, open source, cross-platform, and I have more faith in it remaining available and compatible long term than Bitlocker or the manufacturer-branded stuff.
I suggest you download the
portable version of Veracrypt and play around with it awhile to see what you think. The portable version doesn't need installation, so you won't have to uninstall it if you later decide not to keep using it.
Veracrypt also allows the option of encrypting just a container file instead of the entire external drive, which is great for experimenting or familiarizing yourself with the product without having to commit to encrypting the whole drive until you're ready.
Download and unzip it, then launch Veracrypt. Start by creating an "encrypted file container" of, say, 5 GB. This creates a 5 GB file on your external drive that can be "opened" and used just like a separate 5 GB drive. When you're done experimenting, you can simply delete the 5 GB file from Windows Explorer and everything is gone. Also play around with creating larger or smaller containers and copy the container file from one place to another. Appropriately sized containers are handy for copying onto USB flash drives to carry around sensitive documents when desired.
Once you get the hang of it, you can create a very large container and store it as a very large file on your external drive, or you can alternatively use Veracrypt's Volume Creation Wizard to turn the entire external drive into an "encrypted non-system partition".
