I am not looking for a modern NAS solution with hard disk(s) you have to add manually. I want something like WD My Cloud that is hopefully just plug and play.
So you aren't looking for a good NAS, you are just looking for a particularly shitty one. Now that you've found it what's stopping you?
How can we tell? You need to get at the minimum the kernel logs and something like badblocks and smartctl -a.
Additionally it's probably 10+ years old? Yea, possibly it's gone.
You need to define what you mean by "decrypt", if you mean that you need to somehow tell when mounting what passphrase/secret key you used to the OS and all the operations then with that disk will encrypt/decrypt data on the fly, sure, this is why you bother with it.
If you mean that you have to wait overnight (or even days) for the disk to get decrypted, and then it'll be all clear text, no, that shouldn't be happening.
You have the new one probably, the air (=cheaper to make, that's the idea) one, the helium one is the older. It's absolutely unacceptable what they're doing, and now this cancer moved to the SSDs as well and there it's worse because there are many more ways of putting things together and cutting corners.
DHer favorite, get a NAS (DIY or Synology)! And a cloud for encrypted off-site backups.
Hard drives only write and read data, and everything is tested when you do such operations. It's not like the engine is working but not the radio or I didn't test the WiFi and the antena is busted or something.
With SanDisk it's taking around 8hours to backup 1TB.
That isn't because of the SanDisk, I have Samsung spinning drive (yes they were making that) that's 15 years old or more, over USB2 and it's faster than that.
Get a Samsung T7 or whatever Samsung SSD they have now, but one with metal case not the rubbish rubber ones. Or DIY if you want it slightly cheaper and to pick yourself the SSD and the enclosure.
If they are hot maybe yours are air, these are helium. Yea, leave it to WD to name the same what are as different drives as they could make, heck what two different companies like HGST and WD could make even if one bought the other.
Use rclone, with a cheap VPS if you need. If it isn't rclone supported it's a good sign you shouldn't be touching that service, even if you don't know what rclone is and you didn't ever want to use it.
Now? This is technology from before 2010. If anything they'll be cutting more corners than the usual just to shave a few cents on a niche technology, like they're doing with the optical media.
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Single port but how many disks? More than one? Then make sure you have the appropriate controller.