How does Time Machine handle disconnected external media drive ?

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macked Author

Level 1

8 points

How does Time Machine handle disconnected external media drive ?

I have an external drive connected to my iMac, and backup to a Time Capsule.

I prefer not to keep that external media drive connected all the time...

When I disconnect that external media drive and Time Machine runs, does TM perceive this as "deleted"?

With that drive disconnected, does TM then "release" the prior media drive backup and replace it as it needs space?

When I reconnect the external Media Drive, does TM see it as "new" or recognize it as the prior data and (presuming it's not over-written it) simply manage it incrementally?

Ultimately - do I need to either ensure that the external media drive is always available to use TM, or do I run the risk of screwy / uncertain behavior if it is sometimes present, sometimes not.

Gosh I hope that was clear for you experts out there ;-)

D

iMac (27-inch Mid 2011), Mac OS X (10.6.8), 16 GB RAM, OWC SSD

Posted on May 2, 2015 10:23 AM

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Answer 1

Best reply

Bob Timmons

Community+ 2024

Level 10

159,164 points

May 2, 2015 1:00 PM in response to macked

When I disconnect that external media drive and Time Machine runs, does TM perceive this as "deleted"?

No. Time Machine perceives the drive as not being there, so it does not attempt to back up a something that is not there.

With that drive disconnected, does TM then "release" the prior media drive backup and replace it as it needs space?

No

When I reconnect the external Media Drive, does TM see it as "new" or recognize it as the prior data and (presuming it's not over-written it) simply manage it incrementally?

When you reconnect the drive, Time Machine will back up the changes that have occurred on the external drive since the drive was last backed up.

It might be a good idea once the drive is connected and a backup has completed, to go into Time Machine maybe once a week or so and take a quick look at things to make sure that the external has backed up any recent files that you have added to the drive.

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Answer 2

macked Author

Level 1

8 points

May 2, 2015 1:00 PM in response to Bob Timmons

Thank you. Please let me try to deepen my understanding here.

If I extend your explanation, then an external drive backed up to a TM would forever be retained in the TM... never flushed, as TM discriminates between files that are deleted (and need not be retained and can be removed to make room for new backups) and those that are simply of a missing drive.

Is that consistent with your explanation and understanding?

Thanks again,

D

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Answer 3

May 2, 2015 1:31 PM in response to macked

an external drive backed up to a TM would forever be retained in the TM... never flushed,

Correct. Except.......if the backup drive is close to becoming full, Time Machine will start to delete the oldest backups to make room for new backups.

So, if you have been backing up the external drive for the last year, and the backup drive is running out of space, you may not be able to go back a full year in Time Machine to see exactly how the external drive looked a year ago. You might only be able to go back 10 months, for example, instead of 12.

Most users really don't ever need to go back more than a few weeks or maybe a month or two to pick up a file that they might have accidentally deleted, so the fact that you might not be able to go back a full year or more to see how the drive looked at that time in the past, is not really that important for most users.

Most of us need to know that we have a complete, up to date backup of the external drive in case it fails.

Time Machine will warn you whenever it starts to delete the oldest backups, so if you really do need to be able to go back a year, or two years, or however long it has been since you originally started to back up the external......and you don't want Time Machine to start deleting the oldest backups of the external to make more room.....then you will need to add another backup drive and start backing up everything to that new drive.

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