if you need ultimate portability, but some reliability, maybe check Lacie Rugged Raid Pro 4TB - it has 2x2TB inside which can be configured as either RAID 0 (250MB/s speed) or RAID 1 (mirror backup)

 

Buy what you can afford to back up.
If you trust compressed backups you have more leeway, but my rule is to have an equal size backup storage for everything.
I have a main 8T drive for all archived programs and data as archives (compressed), and a mirror backup on external 8T 5400rpm.
I have a 5T as main backup for OS, media & personal stuff, and an external 5T 5400rpm to mirror that.
I like externals because they can be turned off easily and just turned on for backups.

I am in the market for another external backup drive, because I do not have a mirror backup of my Daw Library drives.
All the libraries are archived and with a mirror drive as backup, but extracting them and rebuilding will take a lot of time...
When I get another drive I will let the archived DAW Library folders go, and just mirror the uncompressed library drives.
Then if (when) one fails there will be no time invested in rebuilding.
Most of that data doesn't change often either.

I already do separate mirror backups of any DAW library folders that I customize or ones that have lots of add-on packs.
There is little chance I would reinstall them and rename them how I have them now, and that means projects would be looking for presets.
So those are almost as important as the project files themselves, and I've always kept mirror backups of those library folders.

 

And a lot of the negative reviews are many times from competing sellers!

I can highly recommend Crucial drives. There is a UK and a US site.