In article
l-september.org, Bill
W wrote:

But is all that really needed, given that there are huge memory
cards (1TB) and the professional will backup the images on the
computer anyway?
the backup protects against card failure.

if you're writing to one card and it fails, you won't have any images
to back up on the computer.

with a second backup card, you will.

it's basically a raid-1.

2 slots is like an extended warranty; most experts in commerce will tell you
it's a waste of money, and the math proves it.

100% correct. Until you lose a wedding shoot, or something else, where
³shoot² becomes what the client does to you.

or worse, having to deal with an angry bride and her family...