How do I set Time Machine to alternate between two attached external drives (so that if one fails I will have a separate backup to use)?

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Level 1

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How do I set Time Machine to alternate between two attached external drives (so that if one fails I will have a separate backup to use)?

Hi Everyone.

Thanks in advance for your help.

Right off the bat let me say I am using Mavericks 10.9.1 on a 2011 Macbook Pro and using all Lacie external hard drives.

I am trying to get Time Machine to alternate between two attached external hard drives, trying to be safe in case one of the backup drives itself fails.

I have both drives set in Time Machine.

From what I gather Time Machine should by default alternate between drives when backing up (I would assume it does one big initial backup on each then subsequent backups are just changes).

But this is not how it seems to be working and I cannot find a straight answer despite much Googling and Foruming

Unfortunately it seems to only be backing up to "External Hard Drive 1".

There have been 4 backups since I set it up, all on "External Hard Drive 1", even though "External Hard Drive 2" is also attached and set in Time Machine.

I manually clicked "Back Up Now" and it started to work on "External Hard Drive 2" (like I want it to) for a minute but it appears to have stopped and gone back to backing up to "External Hard Drive 1".

Am I missing something? Does Time Machine only move on to another disk when the first one runs out of space? Do I have to turn one off in order to force it to back up to the other drive (that would be less convenient of course)?

I am hoping that I am missing something and that it will just alternate between the two. Otherwise it is not the most comforting and thorough backup system as I had hoped when I purchased the additional external hard drive.

I have a 1TB drive and a 500gig partition on another drive, both dedicated to backups (my Macbook HD is 475gig). It will take awhile for each to fill up and who knows what could happen during that time. Having Time Machine alternate between hard drives would be a nice small bit of security if the worst happened.

I realize that there are tons of other options for backing up but I just want to use Time Machine if I can.

Again thank you for your help!

MacBook Pro, OS X Mavericks (10.9.1), Time Machine

Posted on Jan 30, 2014 3:42 PM

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Answer 1

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Kappy

Level 10

361,741 points

Jan 30, 2014 3:48 PM in response to sunwukung

You cannot use Time Machine in that way. I'm not sure where you heard that Time Machine alternates between drives automatically. What you can do is create a mirrored RAID array for the backup. This automatically duplicates whatever is on one drive onto the other drive.

Both drives must be the same size, and prefereably, exactly the same make and model. RAIDs can be configured using OS X's Disk Utility.

RAID Basics

For basic definitions and discussion of what a RAID is and the different types of RAIDs see RAIDs. Additional discussions plus advantages and disadvantages of RAIDs and different RAID arrays see:

RAID Tutorial;

RAID Array and Server:

Hardware and Service Comparison.

Hardware or Software RAID?

RAID Hardware Vs RAID Software - What is your best option?

RAID is a method of combining multiple disk drives into a single entity in order to improve the overall performance and reliability of your system. The different options for combining the disks are referred to as RAID levels. There are several different levels of RAID available depending on the needs of your system. One of the options available to you is whether you should use a Hardware RAID solution or a Software RAID solution.

RAID Hardware is always a disk controller to which you can cable up the disk drives. RAID Software is a set of kernel modules coupled together with management utilities that implement RAID in Software and require no additional hardware.

Pros and cons

Software RAID is more flexible than Hardware RAID. Software RAID is also considerably less expensive. On the other hand, a Software RAID system requires more CPU cycles and power to run well than a comparable Hardware RAID System. Also, because Software RAID operates on a partition by partition basis where a number of individual disk partitions are grouped together as opposed to Hardware RAID systems which generally group together entire disk drives, Software RAID tends be slightly more complicated to run. This is because it has more available configurations and options. An added benefit to the slightly more expensive Hardware RAID solution is that many Hardware RAID systems incorporate features that are specialized for optimizing the performance of your system.

For more detailed information on the differences between Software RAID and Hardware RAID you may want to read: Hardware RAID vs. Software RAID: Which Implementation is Best for my Application?

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Answer 2

sunwukung Author

Level 1

4 points

Jan 30, 2014 4:02 PM in response to Kappy

Thanks Kappy.

This sounds a bit beyond my personal needs... I was just trying to do a little extra to make Time Machine more robust.

Here are some links to places I saw such functionality as I inquired about suggested :

"When multiple backup drives are available, Time Machine uses a basic rotation scheme. First, it checks for any backup drives that are connected to and mounted on your Mac. It then examines each drive to determine if there is a Time Machine backup present, and if so, when the backup was last performed."

- http://macs.about.com/od/backupsarchives/ss/Time-Machine-With-Multiple-Drives-Ho w-To-Use-Two-Or-More-Backup-Drives_2.htm

Effective with Mountain Lion 10.8.x, Time Machine can automatically back up to two (or more) separate destinations, in "rotation." Each backup is completely separate and independent from all others. Backups are run hourly, but each available drive is backed-up to in turn. Note that the first backup to each volume will be a full one; all others will be of everything that's changed since the previous backup to that particular destination.

These are "redundant" backups -- the same set of exclusions applies to each one. You cannot back up different things to different disks this way. Some folks do this already, but prior to Mountain Lion, had to remember to make the switch manually, via the Select Disk button.

- http://pondini.org/TM/34.html

This article could be out of date (indeeed it doesn't appear to be Mavericks they are using) or they could be just flat out wrong (if so I would like to email the author).

Is there any official Apple docs about how the heck Time Machine works?

Thanks again for your help I appreciate it.

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Answer 3

sunwukung Author

Level 1

4 points

Jan 30, 2014 4:19 PM in response to Kappy

I should also mention that it did indeed initially alternate between the two at first, but never finished the backup on the other drive.

There is a backup file on "External Hard Drive 2" labeled "2014-01-30-183016.inProgress" but nothing is in progress in Time Machine and there have been a few backups on "External Hard Drive 1" since that time.

Thanks again.

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Answer 4

Jan 30, 2014 4:41 PM in response to sunwukung

Hi

It definately works, I have my Macbook Pro setup to backup to Time capsule and my Windows server. I didn't do anything special just added Time capsule first and did an intial backup then added my server and did a manual backup - it has been doing alternate backups for over a year. Still does it in Mavericks since it's release.

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Answer 5

sunwukung Author

Level 1

4 points

Jan 30, 2014 4:50 PM in response to simpson53

You are correct! it is now working for me on the other drive... not sure what was going on. But glad it is working as I need it to. Thank you everyone for your help!

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Answer 6

sunwukung Author

Level 1

4 points

Jan 30, 2014 4:52 PM in response to Kappy

Kappy, thank you very much for your in depth suggestions, but I think you are incorrect. Time Machine is indeed automatically alternating between attached drives for me as I had hoped. It is doing the big full backup on my secondary hard drive as we speak. Of course I understand RAID is probably the more "pro" way to go. Thanks again!

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