Can I use two different drives?

Quite the opposite. Use drives from as many different manufacturers as you can, especially when buying them at the same time. You want to avoid similar lifecycles and similar potential fabrication defects as much as possible, because those things increase the likelihood that they will fall close to each other - particularly with the stress of rebuilding the first one that failed.
- To the best of my knowledge, this "drives from the same batch fail at around the same time" folk wisdom has never been demonstrated in statistical studies. But, I mean, mixing drive models is certainly not going to do any harm.
  
  mixing drive models is certainly not going to do any harm
  It may, performance-wise, but usually not enough to matter for a small self-hosting servers.
  
  If everything went fine during production you're probably right. But there have definitely been batches of hard disks with production flaws which caused all drives from that batch to fail in a similar way.
  
  I know it's only what I've experienced but I've been on a 2 weeks of hell from emc drives failing at the same time because dell didn't change up serials. Had 20 raid drives all start failing within a few days of each other and all were consecutive serials.
- If I had a dollar for every time rebuilding a RAID array after one failed drive caused a second drive failure in the array in less than 24 hours.... I'd probably buy groceries for a week.
  
  When using drives from the same model and batch?
  
  I don't know if you're talking about the sample of cases you've personally witnessed, or the population of all NASes in the world. If the former, that sounds significant. If the latter, it sounds like it's probably not something to worry about.

You can use different manufacturers, just make sure they are the SAME size and speed. You can also get the same ones from the same vendor, just from different online shops to try and offset getting a bad batch.

I always thought you're supposed to buy similar drives so the performance is better for some reason (I guess the same logic as when picking RAM?) but this thread is changing my mind, I guess it doesn't matter after all👀
- I heard the reverse, so they don't fail at the same time.
  
  that's also what we did in the early 2000s when building servers. today i don't think it realy matters. i haven't had a failed drive for about 10 years and only needed to swap them out because of the capacity...
- ram matters because the CPU will use the worse speeds and worse timings of all the sticks, drive reads and rights are buffered so it doesn't really matter
  
  Just make sure your RAM has the same timings. Its not a big deal if you have two sticks of each brand

You absolutely can. Of course you'll only be able to use as much capacity as the smallest disk. Sometime ago I was running a secondary mirror with one 8TB disk and 3 disks pretending to be the other 8TB disk. They were 4TB, 3TB and a 1TB - trivial with LVM. Worked without a hitch for a few years till I replaced the three gnomes in a trench coat with another 8TB disk. Obviously that's suboptimal but it works fine under certain loads.
- Oh my god i love that.
  
  Gotta treat for ya:
  The three gnomes in a trench coat are the three from the left.

Hardware or software (BTFRS, ZFS etc...) RAID?
- Software

As long as they're mostly the same. For example on many controllers no mixing SSDs with HDDs.

If you haven't looked into it, and if you already have the disks of varying capacity, check out JBOD. You will have to configure a system for backups however as you wont have parity like raid1
- Raid is not a backup...
  
  I'm aware, but raid 1 is mirroring which is redundancy, a jbod offers no redundancy so a backup would be even more crucial to protecting from data loss. Also i never said raid is a backup.

I usually find the cheapest drives and buy multiple of those, but you should be able to assemble a RAID out of different disks, though you'll be limited to the space of the smallest one in the mirror set.
Also make sure that your RAID systems supports this.
- Okay. Where do you buy the disks?
  
  Ebay. If you're outside the US, you'll probably be better off with a more local site.

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

NAS Network-Attached Storage
RAID Redundant Array of Independent Disks for mass storage
SSD Solid State Drive mass storage
3 acronyms in this thread; the most compressed thread commented on today has 15 acronyms.
[Thread #328 for this sub, first seen 2nd Dec 2023, 20:35] [FAQ] [Full list] [Contact] [Source code]

Fewer Letters	More Letters
NAS	Network-Attached Storage
RAID	Redundant Array of Independent Disks for mass storage
SSD	Solid State Drive mass storage

I would strongly recommend that you get the same drive. It doesn't make any sense not too