I am not going to explain file synchronisation to you.
You can read about it here: https://en.wikipedia.org/wiki/File_synchronization
What I will talk about is how using a tool like my old friend Allway Sync can help you manage the thousands of files and photos you invariable have.
A tool like Allway Sync allows you to create rules on how to copy files around your various hard disks and other storage locations (network, thumb drives, FTP and so on).
Because tools like this analyse the source of the files and compare the results to the target (or destination) of the files, they can decide what subset of files actually need to be copied (as opposed to everything).
This can be a massive time saver. It can save a lot of hardwork and headaches too.
Imagine, like me, you have thousands of photos in your pictures folder.
I am currently at nearly 70,000 pictures at 151GB in size.
That's nuts and that's something you don't want to be copying around the place too often.
Like me you probably add photos to various folders as you take them and copy them from your camera and/or phone.
Over time there's no way you'll know where all the "new" photos are.
So you options for (manual) backup of all your photos are to a) copy everything b) use a synchronisation tool to analyse and only copy what has changed and is new.
I use Allway Sync to synchronise new and changed files to my external hard drive backups, thumb drives of handy tools, documents to my secure OneDrive vault, music to my other computers, information sheets to DropBox, documents to encrypted volumes and so on and so on.
I wouldn't want nor try to do all that manually and my data security would definitely suffer as a result.
Most file synchronisation tools have options like those pictured above.
They'll run on schedules so you don't have to remember to synchronise things manually.
They'll think about things like size changes, dates and times of creation and modification.
And also handle or warn you of the things that presents problems (errors) so your human brain can make a choice (if you want).
I have tried a number of tools like this but I end up coming back to Allway Sync.
It looks kinda old and clunky but it works better than the rest.
It comes in a FREE version which limits how much data you can ship around. This may be OK for occasional users and certainly is OK for testing to see if you like it.
But I paid for the PRO version. A one off lifetime fee that feels like extremely good value some 11+ years later.
Have at it. Have fun. Save time. Life is too short to spend copying files.