MacBook – Time machine drive used for time machine before..can it back up a new drive without erasing

Time Machine allows you to backup multiple macs to one drive. You just plug it in, set it up, and you're good. If you look at the file system, you can see separate backups for both computers. At our home we currently do this with a Mac Mini and a MacBook. The Mac Mini is always plugged in, and the MacBook gets updated every couple weeks. We use a 1.5 TB drive from Western Digital. Given your description, I think this solution with your 2TB drive should work well for you.

The data backed up by both macs will not be the same, as no matter how hard you try, both computers will wind up having subtile differences, so the data won't be "exactly" the same. Backing up only one system doesn't seem ideal to me, but you can restore a MacBook from a Mac Mini backup. It works fine.

Also, you realize that the drive must be connected for you to access the photos and music that you have stored there. You'll have to factor that into your storage solution.

Basically, I would recommend not trying to constrain time machine to only a weekly interval on the offsite drive.

That being said, you can have both drives work totally independently as that is how the software is engineered. If you really only want a weekly snapshot, only connect the weekly drive once a week and force a rotation of the destination:

 tmutil startbackup --rotation

Once you have a backup to the weekly drive, eject it and send it away. As long as all dives get reattached every 14 days or so, you won't get a warning dialog that any are out of date.

Again, you can certainly enforce a single weekly backup by the procedure you follow but two things make that practice less needed than you might expect:

• The backup intervals are very low space overhead compared to the size of cheap backup disks.

On my worst Mac, the system averages 5 MB of overhead for system log files so in absence of user files changing, you have 24 daily and 31 monthly snapshots you would avoid storing 275 MB (or 0.0003 of a one TB drive) if you were on a strict one per week schedule.

Since the cleanup of old backups is so fast and the storage impact so low, most clients with offsite rotation end up just swapping disks each week and let the software cull the extra snapshots per the normal schedule, leaving weekly snapshots more than a month old.