VeraCrypt is a free file encryption software.
I use VeraCrypt occasionally to encrypt whole drive partitions. The best thing of all is that VeraCrypt is totally free to use and it’s open source. The way it works is by creating encrypted disk images. That means VeraCrypt can encrypt your entire drive or just be a simple disk image file sitting on your computer. The software mounts these disk images when you enter your password so you can open the drive and access its content. VeraCrypt is one of the few solutions to file encryption that also allows image viewing programs to work properly. Because you mount your disk image, it works just as an external hard drive would.
Another nice feature of VeraCrypt is that it can hide your encrypted partitions too if you wish, or double encrypt something (a disk image inside a disk image).
Some reviewers say that VeraCrypt is more advanced which I suppose it is in some ways. However I still consider it easy to get the hang of for most computer users.
VeraCrypt was based on the abandoned project Truecrypt, which I used to use many years ago and liked a lot. Privacy advocates strongly recommend switching to Veracrypt due to it being more updated than Truecrypt, which now apparently contains several vulnerabilities.
The thing I dislike most about Veracrypt is that you have to specify a size for the encrypted disk image at the very start. This forces you to think about how big you want the drive to be, which can lead to overestimating and wasting disk space. Or conversely it could be that you make the disk image too small and later have to modify it. Whenever you update a Veracrypt disk image, it is treated as one file no matter how many files there are inside it. For that reason, Veracrypt doesn’t really work very well with cloud storage and is best used locally.