I would never even consider using a failing/failed HDD as a backup solution. That is just asking to lose your data.
Solid State technology has progressed a LONG way from when it first came out. At this point, it isn't that much more expensive than a mechanical drive, is MORE reliable, and orders of magnitude faster. The performance improvement by itself makes it worth the investment. After installing one in my laptop, I was so impressed by it, I switched ALL of my machines to solid state drives. (even made my wife happy.
Disadvantage of SSD? If it fails, that's it, yer done. With a mechanical hdd, there is some hope of recovering your data, on an SSD? Nope. It's gone. So, backups are a VERY good idea.
If your data is important to you, more than one backup solution is a requirement. Having a second drive in the same machine is ok, but, if that machine takes a power hit, that smokes the primary drive, or a powersupply failure, or anything similar, it's quite likely that BOTH drives are going to fail...... So, an external HDD, that ISN'T always plugged in is a better idea.
Having an online backup is a very good idea. There are free solutions, but, they don't give you a lotta space. For paid solutions, I rather like Carbonite. Not hideously expensive, fully automatic. Install it once, set it up, and forget about it. Also has the advantage of your files being accessible from anywhere.
As for HDD's, Hitachi, HGST, and Toshiba drives are terrible, and the larger they are, the more terrible they are. I can't count the number of them I have replaced.... and they weren't that old. Seagate is generally pretty good, Western Digital (black, or gold drives.) used to be good, but, they got bought out, and I don't know how good they will remain. The Blue or green drives are best avoided......