It’s probably the cheapest 20TB Seagate

Yes, that the reason I bought it!

I’d check the s/n on seagate’s site but it might be a white label drive (aka a OEM/rejected drive)

Seagate website says “This product was originally sold as a part of a larger system”, so yes, I guess OEM.

I don’t know the bitrate of your cameras, but even if they were high quality 4K, it should be maximumx 16 Mb/s for each camera, so 48 Mb/s in total.

Assuming the device is connected with gigabit LAN, you still have more than 900 Mb/s for your Jellyfin.

Maximum UHD Bluray bitrate is 128 Mb/s, so you can play 7 original quality movies at the same time, without saturing the network.

So, as you can see, from network point of view, it is not a problem.

I would be more concerned about CPU usage, especially if you are re-encoding the security cameras streams (or you are using motion detection), and/or if you are planning to use re-encoding with Jellyfin.

(also, you obviously cannot play 7 movies at the same time if you are using a single mechanical hard drive, it can be done if you are using an SSD, or multiple hard drive in RAID)