Simply connect your external hard drive to your PC, and double-click on the hard drive icon on your desktop to open it, then copy and paste your wanted files into there. After that, all you need to do is to wait for the transfer process to be finished.
Well yeah, that's how I did it prior to LibreElec/KODI when I was just using the built-in TV media player. 
I'm just trying to avoid having to use a second device (laptop/PC) and just utilize the RPi5 for the whole thing and just have a flash drive/SSD in my pocket that I can use. =)
Kodi Settings > File Manager .. it's a little clunky to use but does exactly that ^
Or copy things over the network (SMB or SFTP)
Or realise that 14TB should really be a NAS device on the Network somewhere 
I'd seen the File Manager before, but as you said, looked interesting Unfortunately for this use-case, I don't get network access and a NAS is sadly too large for where it's being stored (and again, no network, so it'd be a direct-connect NAS).
Just to add a little more context, I'm using MediaElch to download the metadata for KODI (artwork, the NFO file data, etc) pre-emptively and including that when I add data to the 14TB HDD. I do this because I know I can't utilize the built-in scraper because of the whole no-network thing.
That being said, if I use the File Manager approach, do I need to do anything else to have KODI "see" the new/updated content that was added?
On different Raspberries I've been using external USB drives with TB and TB of space for music and movies directly connected since 2016. The procedure is much more simpler than the couple you mentioned. The Raspy for USB devices has always been plug and play, with every distro, LE included. If you want to do exactly what I do every 3-4-5 weeks (after several downloads got on my laptop): just detach the USB drive, attach it to the laptop, load whatever you want on present or newer folders, re-attach it to the Raspy and enjoy. Never a problem, and I use NTFS everywhere on my drives for robustness of data, and no need to switch off and on the RPi.
So far my process (as I've only done this once) was to shut down LibreELEC/KODI gracefully using the menu options in KODI, unplug the USB and attach it to a laptop I bring with me, add/remove/update files as needed, then re-attach and turn the RPi5 back on. I noted that KODI did not see the removed/added files, so I had to have it rescan the library (I don't recall the exact term used in KODI, apologies for that).
I don't know how you access the data on the drive... I use the Raspy automatic mounting/dismounting, that put/put away the USB drives under /media/
I added the 14TB drive as a movie source (all files in one folder, not separate folders) and a TV source (these are in separate folders by program/series). As mentioned above, I use MediaElch to pre-populate the metadata for all the files given the lack of a network.
TL;dr: I'm trying to avoid bringing the laptop to do these updates, I'll investigate the File Manager, I suspect I'll need the keyboard for the best experience with that. Hopefully if all the content is in one folder it'll just let me select all and go.
Thank you for all the responses!
