There are several options under File, Backup copy within the program.

The File, Backup copy, Copy project and media to folder is probably the best option of these to copy to external hard drive, however do note that whichever of these options you choose only used media will be included.

If you have other unused media, that could have been used or may be used in a re-edit of the project at a future date, then this will not be copied and will have to be backed up separately.

There are other options depending on how you have organised the project folder and media folders.

HTH

John EB