Piracy: ꜱᴀɪʟ ᴛʜᴇ ʜɪɢʜ ꜱᴇᴀꜱ

For seeding? Nah. Not unless you have a ton of upload and loads of peers making requests.

Get something that has an internal mirror, that'll give you double the read throughput AND some redundancy.

Any recent-ish spinning disk should do. You're probably not going to be seeding fast enough to saturate the read speed on whatever you choose anyway. The real question is how much are you willing to spend? I've seen fairly recent deals for 18tb WD EasyStores for around $250, but that's going to be slightly higher $/tb compared with something like a 12tb or 14tb drive.

How much space do you think you'll need? What type of redundancy systems are you planning to put into place, or even still do you plan to put any redundancy systems in place? Is there a specific reason you'd like to go with an external hard drive over setting up an internal raid array of some sort? Was your plan simply to just have an external drive plugged into your pc through USB or USB-C?

I think these would be my main questions before I could feel comfortable pointing you in any directing, since naming a brand name or specific drive while efficient probably wouldn't be good for you in the long run.

Spinning rust is fine, if you seed a lot, you will often be the only seeder on a torrent. In that case being able to keep the file around is fare more important than shaving milliseconds off download times.

SSD is good for low amounts (1TB-2TB) (they are really cheap rn) but if you need more then try to get a good deal on 18-22tb externals.

A spare old computer with something like freenas or similar should be plenty. For simple seeding disk IO is hardly going to be an issue unless for some reason your internet upload managed to exceed the disk speed (it wont) so anything with a low energy footprint would do fine. A raspberry pi with any old USB plugged into it is plenty. Larger issue is being able to rout the traffic over a tunnel to keep prying eyes in their own business.

buy a bare hard drive with a very long warranty period and buy a separate enclosure and just put it in the enclosure. the only exception I ever made to this rule was those WD black hard drives that come in an enclosure, since I have bought a number of bare WD black hard drives and never had one fail.

My advice it's to buy a cheap Netac USB 3 SSD at AliExpress. they work pretty well tbh.

I just recently got fed up with this stupid “WD MyCloud” box (AVOID) and stripped the drive out and put it in a $20 USB drive adapter, which works PERFECTLY.

This is the drive adapter: https://a.co/d/1BdB0Oa

So my recommendation is one of those plus whichever hard drive is priced right. There is a website that tracks hard drive prices in dollars per gigabyte, not sure of the address right now.