When transferring data to a new laptop does your older laptop keep the data or does it erase?

It gets "cloned" which generally doesn't affect the source device. They can basically be "branched" where each one is more or less operating independently.

There are some caveats, such as the use of iCloud sync. I wasn't sure and my Safari bookmarks got copied over to a work computer that I set up with my own Apple ID. But when I tried to clear the bookmarks, that synced with the "donor" and cleared. I turned it off and then recovered some some saved bookmarks that I created.

Your data will remain on the old computer unless a problem occurs. For best results, create a Time Machine backup or bootable clone and migrate from that.

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