Understanding encrypted and unencrypted MacBook Air backup

When I started backing up my MacBook air with my WD drive, I got a message "you are backing up an encrypted disc with an unencrypted disc". What does this mean? Is this bad?

All of this assumes you weren't aware that FileVault had been enabled on your MacBook when you upgraded to Tahoe.

If you weren't and you didn't have FileVault enabled, it is now. That triggered that warning that you wouldn't have seen before.

If you don't need FileVault enabled on the Air, turn it off (Privacy & Security System Settings). For my MacBook I turned it on in case it gets stolen (a little harder to get to the data, but not much). All modern Macs are encrypted all the time. FileVault just changes how the decryption keys are handled. If you have your Mac set to automatically log in, then the encryption won't stop someone from getting to your data. FileVault disables automatic login.

Even though I use FileVault on my MacBook, I don't encrypt the backup because I don't want another barrier to restoring my data should I need to do it. My backup drive is relatively safer at home than my laptop when out in public.

the purpose of this message is to let you know that, as it states, you are backing up an encrypted disk (your MacBook's SSD) with an unencrypted disk (your external drive). This message exists to avoid someone accidentally backing up otherwise secured, encrypted, confidential information on an unsecured hard drive.

So, simply put, you have your local storage on your Mac, which can not be read or in any way used unless you are signed in. backing up your local storage on a disk, with an unencrypted backup, said data which you have on your Mac will be on that backup, just unencrypted, in this case, so, can be read by anyone that has access to the hard drive.

I hope this way it's more understandable.