Replacing MacBook's SSD with a new one - how to clone?
If you can't connect the old and new drives to the Mac at the same time, follow this procedure:

Create a clone copy from the old drive onto the intermediary external USB drive, using Carbon Copy Cloner or SuperDuper!.

Then swap the hardware over.

You will need to boot to the intermediary drive (or a fourth drive!), so that you are free to write all over the new internal. (So make sure you've installed an OS on the disk beforehand...!)

Use one of the apps mentioned above (the same one!) to copy the clone back to the blank new drive.


Generally, I would always advise having a spare drive with a bootable OS on it, 'just in case' - even if it's just a little 64 GB flash drive or something. (Oh, and of course, you'll also have another drive for your backup...!)