One copy on the machine = not backup.

Two copies is still not a real backup since when we lose the first copy then we are back to no backup.

Three copies and we are in a state of backup that works.

Many make this mistake or worse, use those externals for storage which if you are reading this forum you see folk asking how to recover their files daily.

As to brand, I have yet to find any that are better than average but having said that, all have worked very well while none have survived the 3 foot drop to the pavement.

I know this may not answer your question but I hope you can find a way to backup what you don't want to lose.