I've just added an external hard drive to become a new Time Machine disk, with two mirrored partitions. I've disconnected my older TM drive (it's fine). Will the next backup, to the new drive, automatically be a full capture of my iMac, or just another snapshot? I need to be sure what I'm archiving in prep for using the new drive as the source for migrating to a new iMac.
iMac (2017 – 2020)
The first Time Machine (TM) backup to a brand-new disk is always a full backup, not just a snapshot. Since this external drive has never been used by TM before and the previous TM disk is disconnected, macOS has no backup history to reference. As a result, the initial run will capture the entire contents of the iMac—system, applications, user data, and settings—creating a complete baseline archive.
After that first full backup completes, subsequent backups to the same disk become incremental, recording only changes since the last backup. Local snapshots may still exist on the internal drive, but they are irrelevant to what’s written to the new external disk. When preparing for Migration Assistant on a new iMac, this first backup is exactly what you want.
As a best practice, I suggest that you wait for the backup to finish fully and then verify it by entering Time Machine or selecting the disk in Migration Assistant to confirm it’s recognized as a valid source.
The first Time Machine (TM) backup to a brand-new disk is always a full backup, not just a snapshot. Since this external drive has never been used by TM before and the previous TM disk is disconnected, macOS has no backup history to reference. As a result, the initial run will capture the entire contents of the iMac—system, applications, user data, and settings—creating a complete baseline archive.
After that first full backup completes, subsequent backups to the same disk become incremental, recording only changes since the last backup. Local snapshots may still exist on the internal drive, but they are irrelevant to what’s written to the new external disk. When preparing for Migration Assistant on a new iMac, this first backup is exactly what you want.
As a best practice, I suggest that you wait for the backup to finish fully and then verify it by entering Time Machine or selecting the disk in Migration Assistant to confirm it’s recognized as a valid source.
Time Machine: Full backup or snapshot on new drive?
