Xbox one 2tb seagate external storage has stopped working what do I do ?

Start here for basic troubleshooting steps with external drive detection: https://support.xbox.com/en-US/games/troubleshooting/console-doesnt-detect-external-storage

Put your ear up to the drive while it's powered. Is it making constant, repetitive patterns of noises (hard clicks, whirrs, etc)? That might indicate that the drive has died. If it has died, you will want to look into

Seagate's website about getting it replaced under their warranty, if that's still possible.

If you aren't hearing those noises and it sounds normal, you can try unplugging and replugging it in your Xbox, to see if it gets detected. You can also perform a simple
power cycle and see if the drive gets detected again. If the data is corrupted but the drive still works, check if it's still detected when plugged into a Windows computer and see if you can reformat it (use
NTFS as the file system) - note: this deletes everything off of the hard drive. Then, you can plug it back into the Xbox, reformat again (through the Xbox prompts), and reinstall your games from there.

My son is needing an external hard drive (2TB) for his old Xbox 360s. What do you recommend?

Pretty much any brand of external hard drive will work on the Xbox 360. But, I recommend anything from either Western Digital, Seagate or Toshiba should work fine.

My old 360 - which I use occasionally has a 2TB Western Digital external hard drive.

As I said before, it's a good idea to install all disc based games now. These Xbox 360 consoles aren't getting any younger!

i can't get pass step three, as it just takes me back to step two. by that, i mean that a data error pops up (error: data error (cyclic redundancy check)).

Coincidentally I too had a similar problem with my 2TB Seagate external. I was running it on the back of my Xbox One X. Every once in a while my Xbox wouldn't load right upon powering up. Unplugging the drive and starting up without it plugged in always
fixed the problem. Then the Xbox eventually stopped recognizing it all together. I followed the same disk management steps Oynx posted above (that's great advice) and it worked for me until it happened again. Then I ran disk management again and fixed it.
Worked for another month or so and the Xbox stopped recognizing it again. It was a pain in the neck because I always had to reinstall the games but then I couldn't even get my PC to recognize it. The same disk management steps that always worked
just don't anymore. For me I suppose it's time to replace the drive.

i can't get pass step three, as it just takes me back to step two. by that, i mean that a data error pops up (error: data error (cyclic redundancy check)).

i tried to fix it on my PC, but i get stuck due to the cyclic redundancy check data error.