I think the external hdd's are not ventilated or cooled properly, that makes them hard for use for 24x7 usage. I use RPi4 but only for plexamp. But for movies and all, a cheap DIY system running the Xpenology can do wonders. Yea, it might take some more amount that a simple hdd on a RPi4 but, it comes with its own advantages. I also use Asustor 4 bay nas but the Xpenology system has more power than any NAS out there. Now for your answers:
1) hdd's might be getting hot due to 24x7 use.
2) Better RMA, also OVM has the plugin that lets you put the hdd's on sleep, you can try that.
3) Warranty is from the initial hdd onwards, any RMA units warranty still applies from original purchase date.
4) not sure
5) Always delete the movies/tv shows that you are sure may not watch them again in an year. Now with good broadband speeds, its better to redownload them instead of keeping them and adding storage...
