FWIW - an option is to simply copy your *.ftm file and related FTM media folder to your backup storage device.
That is, if you have always selected the option to ATTACH media to items when merging - rather than LINKING them to other places on your hard drive.
It is advisable to have all of your media in the FTM media file for backup purposes and to avoid any links getting disconnected to FTM due to files being discarded, moved, etc.
If you don't have all of your media inside the FTM media folder, you can also backup your entire "Documents" library/folder at once - which in this day and age is not a problem to do, with all of the gigantic and cheap external hard drives and cloud backup alternatives available.
That is, if you have always selected the option to ATTACH media to items when merging - rather than LINKING them to other places on your hard drive.
It is advisable to have all of your media in the FTM media file for backup purposes and to avoid any links getting disconnected to FTM due to files being discarded, moved, etc.
If you don't have all of your media inside the FTM media folder, you can also backup your entire "Documents" library/folder at once - which in this day and age is not a problem to do, with all of the gigantic and cheap external hard drives and cloud backup alternatives available.