[Robin S. Kurz] “But rather due to the fact that if IT goes, it ALL goes.”
Same can be said for your USB 3 drive. It should have a backup, period. Editing from your main SSD doesn’t put you in any more danger other than having a backup.
Frankly, that reason is rather ludicrous. That’s like saying that you should have a different car for when you drive to work and when you drive for pleasure, because if your work car gets totaled, then you would be in it.