What am I going to do, put the $20 back in my pocket? No, that’s a dick move. I see that they’re acting like they’re in a rush. Pocketing the paper money will only slow down the transaction.

It’s a lose-lose scenario, just like the grandparent commenter suggests.

How do I know the total until I’m given it?

The store knows which items are taxed and which ones aren’t.

I get you, but cashiers are trained to be impatient and never wait for me to pull my change purse out to round it up, even if I tell them there’s more coming.

This is how it typically goes:

Cashier: Your total is $10.50. Me, handing them $20: Here, hang on to this, I’ve got some change. Cashier snatches the money, enters $20.00 in the machine and stuffs it in the drawer as I fish out the appropriate change. Me, as they are in the midst of gathering a large amount of change from the drawer: Here you go. Cashier, interrupted during their change counting, furrows their brows at me as if I just tried to pull a fast one on them.

It can’t be a quick change scheme if you haven’t given me my change yet. Just don’t be in such a rush.

I have no experience in this specific matter, but you could look up how to switch the sector size from 512 bytes to 4096 and, you know, just do the opposite.