Even with a 256-bit key, Apple is not recommending that FileVault be used for encrypted backups or files on external disks, which could benefit from a faster key generation process. FileVault is primarily intended for encrypted drives and files on the local disk. When I set up a FileVault-encrypted external drive and mounted it to a remote network, I had to make sure that both my local computer and the remote machine were running OS X Yosemite, since the file system on the remote machine was NTFS. That's something a computer administrator ought to be doing as well. I found that Apple's recommendations of a key size of 128 bits or longer was sufficient for my purposes. The first thing you need to do is create an encrypted disk image with the macOS FileVault utility. This can be done by clicking File > New > Disk Image (or, more conveniently, double-clicking the disk in the Finder.) This image is then used to create the hard drive of the file system. Encrypting a USB Flash Drive What if I need to encrypt the data on a USB flash drive? Apple makes it very easy to encrypt a USB flash drive by using Apple's own utility, Disk Utility. This utility (download here) will encrypt the entire contents of a USB flash drive. Dec 13, 2023 — I've written a guide to using Disk Utility to encrypt a flash drive. It's not a detailed guide, but it is a great tutorial for beginners and, more importantly, shows how to use Disk Utility to encrypt your first file. Use Disk Utility to encrypt a flash drive — Ask Different If you need to use one of the Apple built-in features to encrypt a USB flash drive then the best thing you can do is to go to the Disk Utility (on macOS here or on Windows here) application and create an encrypted disk image (file) on the system hard drive. This is called a “disk image.” Just click on Create an Image Disk ... (or Disk Utility will prompt you to create the image on the drive you want) and follow the prompts to create a disk image of the disk you want. When the image is created, just close Disk Utility. Now that the image is made, it can be mounted just like a regular volume. Just click on the volume you want to mount. When in Disk Utility, click on the View tab and then click on the Advanced Tab.