is time machine backup of encrypted MacBook to unencrypted disk safe?

DLG52 wrote:

I appreciate that! I'd thought that encryption wasn't necessary, but I'd using time capsules connected via wifi prior to this year. Time machine on external drive via usb-c is a new experience.

I’d consider Encryption necessary, as sooner or later all storage will be replaced or retired, and it is not always possible to erase the contents of storage when some failure or some retirement arises. With encryption using a robust password, whatever dats is left on the failed storage is still protected. Hard disks in particular can fail such that hunks of stored data are inaccessible, and other hunks are entirely readable, and none of it is reliably erasable.

DLG52 wrote:

1. I have a MacBook air with Venura OS and purchased a Sandisk Extreme SSD (1 tb) to replace existing time machine backup disk. I've seen online Mac helps that say no need to encrypt the disk, but when I started formatting my Extreme SSD I got warnings that it's unencrypted. Two questions:

Does Time Machine maintain encryption of my MacBook OS?

Time Machine manages the encryption of the backup. Not of the Mac.

2. I want to transfer the Time Machine backup data on existing drive to Extreme SSD: how should I do that?

That copy operation doesn’t work all that well. The backups are very slow to copy, based on how they are structured on the backup volume, too. You’re probably better off starting a new backup, and retiring the old backup device when you’re done.

There are some existing discussions of the struggles that can be involved.

Moving a time machine backup - Apple Community

(At least one of the “how to move the backup” discussions I’ve recently located doesn’t even do what it claims, and simply starts a new backup.)

Also: Some Sandisk Extreme SSDs have issues: https://support-en.wd.com/app/firmwareupdate

Adding to MrHoffman's replies: if you choose not to encrypt a Time Machine backup, that backup can be used to effectively create a duplicate of the Mac it backed up, without requiring a password. That is its purpose.

Strictly speaking there is no need to encrypt it. Time Machine doesn't care.

You can also connect any external USB hard disk drive directly to a Time Capsule. It just needs to be formatted Mac OS Extended (Journaled) first. That way, it won't have to be connected directly to your Mac and Time Machine will work exactly the same way it always has: wirelessly.