I would like to know how long such an external HDD from WD lasts! (1TB)
And whether it's smart to run games like Fortnite on it.
3-5 years for sure. Depending on how you treat them, significantly longer.
External hdds are usually quite slow. The length of life depends on how you treat them. I would either buy an external ssd or an internal hdd
It is in operation about 8 hours a day!
Have one that lasts for 20 years