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Acronyms, initialisms, abbreviations, contractions, and other phrases which expand to something larger, that I've seen in this thread:

6 acronyms in this thread; the most compressed thread commented on today has 30 acronyms.

[Thread #542 for this sub, first seen 25th Feb 2024, 08:45] [FAQ] [Full list] [Contact] [Source code]

I believe ZFS works best when having direct access to the disks, so having a md underlying it is not best practice. Not sure how well ZFS handles external disks, but that is something to consider. As for the drive sizes and redundancy, each type should have its own vdev. So you should be looking at a vdev of the 2x6TB in mirror and a vdev of the 2x12TB in mirror for maximum redundancy against drive failure, totaling 18TB usable in your pool. Later on if you need to add more space you can create new vdevs and add them to the pool.

If you're not worried about redundancy, then you could bypass ZFS and just setup a RAID-0 through mdadm or add the disks to a LVM VG to use all the capacity, but remember that you might lose the whole volume if a disk dies. Keep in mind that this would include accidentally unplugging an external disk.

You’ve got some decent answers already, but since you’re getting interested in ZFS, I wanted to make sure you know about discourse.practicalzfs.com. It’s the successor to the ZFS subreddit and it’s a great place to get expert advice.

Is there any particular reason you're interested in using ZFS?

How do you intend to move over the data on the 2x6 array if you create a new pool with all the drives?

mdadm RAID1 is easy to handle, fairly safe from write holes and easy to upgrade.

If it were me I'd upgrade to a 2x12 array (mdadm RAID1 or ZFS mirror, whichever you want), stored internally. And use the old 6 TB drives as cold storage external backups with Borg Backup. Not necessarily for the media files but you must have some important data that your don't want to lose (passwords, 2FA codes, emails, phone photos etc.)

I wouldn't trust USB-connected arrays much. Most USB enclosures and adapters aren't designed for 24/7 connectivity, and arrays (especially ZFS) are sensitive to the slightest error. Mixing USB drives with any ZFS pool is a recipe for headache IMO.

I could accept using the 2x6 as a RAID1 or mirror by themselves but that's it. Don't mix them with the internal drives.

Not that there's much you could do with that drive setup, since the sizes are mismatched. You could try unraid or snapraid+mergerfs which can do parity with mismatched drives but it's meh.

Oh and never use RAID0 as the bottom layer of anything, when a drive breaks you lose everything.

Make sure you understand volume block size before you start using it. It has a big impact on compression, performance and even disk utilization. In certain configurations you may be surprised to find out as much as 25% of your disk space (in addition to parity) is effectively gone and it is untrivial to change the block size after the fact.