Do you want to encrypt an external hard drive or USB flash drive connected to your Mac? Encrypting the drive will protect the data on that drive. Each time that drive is connected to another Mac you’ll need to enter a password before the drive is available and before you can view or edit the files on the drive. Apple makes it easy to encrypt an external drive.The feature is built into macOS, the Mac operating system. Right-click (or control-click) on the drive’s icon on your Mac’s Desktop (or in the left-hand sidebar of a Finder window) which causes a menu to appear.
Select Encrypt <Your Drive’s Name> from this menu. Then a window will appear prompting you to type in a password which will be used to encrypt the drive. In the Verify Password field type in the encryption password a second time. Type in a hint to remind you what the password is. Finally, click the Encrypt Disk button. Sit back and wait. Depending upon the capacity of the drive and the amount of data on the driv, the encryption process could take anywhere from a few seconds, if the drive is empty, to many hours or even a day or longer if it’s a large capacity drive with a lot of data on it.
Please note, the Apple requires you to enter a hint. Why do they do this? Because, if you forget the password then you’ll lose access to all of the data on that drive. I’m serious. There is no back door. There is no alternative way to break through the encryption and get to your data. So, please leave yourself a good hint but also add this password to your password list.
If the encrypt option is not available then your drive might not be formatted as a Mac drive. You can check the drive format easily, control-click (or right-click) on the drive’s icon and select Get Info. Then read what’s listed in the Format section. If it reads Mac OS Extended or Mac OS Extended (Journaled) then it’s a Mac formatted drive. If it reads something like FAT, NTFS or exFAT then it’s not a Mac formatted drive. You could re-format the drive but think before you do this. Reformatting the drive will erase the drive so you’ll need to copy your files off of the drive first. Also, be aware that Macs can read both Mac formatted and PC formatted drives, but PCs can only read PC-formatted drives. So, if you need to use this drive on a PC you don’t want to reformat it.
it’s possible to setup an encrypted drive that can work on both Macs and PCs but it’s more involved. You’ll need to install a third-party encryption application like VeraCrypt on every Mac and PC to which you’ll connect the external drive.