How to properly migrate from my external hard drive to a NAS setup?

[4r5hw45twh]

So, I host a couple of websites as well as my media interface (Jellyfin) and to do that, I use a mini PC with Ubuntu on it and a 20TB WD HDD external drive that has the actual media on it (using 18TB of it). The NAS equipment and drives arrive at my house tomorrow.  Each NAS hard drive will be 18TB in size. I plan to use my NAS setup in addition to the server mini PC. I also plan to simply copy-and-paste the 18TB of media from the current external drive to the NAS setup, but how do I do that since the NAS will have 4 different drives in it? I've never used or had a NAS before Because already, one of the drives would be full, so as I put new media on the other 3 drives, how do I tell Ubuntu/Jellyfin where to get the media from at that point? I also remember in the past when I had old SSD's and then bought this big 20TB HDD to store it all on one drive instead, my entire Jellyfin library got messed up and I had to re-import everything, etc etc - can this be avoided?

[34626]

My current setup is kinda like yours, just with 4 external drives.. So i just bought a case and wil transfer parts from another PC to it, since it can hold up to 12 drives internal.

I use Debian 13, headless (not GUI) and my OS is on a SSD, while only some data is on same drive for now, but that will change, but my libraries is also on other dries that i have mounted in Debian on /mnt/ext1, ext2, ext3, ext4 and first folder then is data in each and then a JMS1, JMS2, JMS3, JMS4 and also a JMS1_2, JMS2_2, JMS3_2 and JMS4_2

JMSX is for read only folders for Jellyfin while JMSX_2 is read and write. So Jellyfin is not able to edit metadata on the libraries that is in production.

Is Jellyfin running in docker? If, then its very easy to maintaine, and i would recommend your to use a systemdrive for it (SSD) so jellyfin is faster and also prevent your HDD from crashing. (Theres a lot of plates inside it, so multiple drives is better then one big when in production) - personally i have 2 8 TB and 2 4 TB drives... The 2 4 TB was some i had lyting around, but i plan on using them for cold backup and also have a 18 TB drive with same purpose when i get the case.

I plan on using this case:
SilverStone Grandia GD08

For backup drives i plan on using OpenZFS (ZFS) and use that to use OpenZFS to copy data.. else i use "sudo rsync -au --progress /source/dri/ /destination/todir/ with screen so i can let it run in the background.