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Dave_O

macrumors regular
Original poster
Nov 16, 2018
112
48
I have a few basic questions about Time Machine.

1. Can I revert to a previous state if a minor OS upgrade has occurred, like 10.15.5 to a time when 10.15.4 was on the Mac?

2. Same question, but for major OS releases, eg can Time Machine go back from Catalina to Mojave, even with the new System Volume that Catalina uses?

One source of my confusion is that Time Machine seems to run in User space, and it's unclear how it could roll back OS components that may be active or privileged.
 

Weaselboy

Moderator
Staff member
Jan 23, 2005
33,183
13,621
California
Yes to both #1 and #2. TM has full admin rights to backup everything, including the OS.

You would just option key boot to the TM disk then erase the internal disk and restore. During the restore process you will get a screen like this. Just scroll down (back in time) until you get the OS version you want and restore.

iu.png
 
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Dave_O

macrumors regular
Original poster
Nov 16, 2018
112
48
Yes to both #1 and #2. TM has full admin rights to backup everything, including the OS.

You would just option key boot to the TM disk then erase the internal disk and restore. During the restore process you will get a screen like this. Just scroll down (back in time) until you get the OS version you want and restore.

View attachment 842877
Thanks, that clears things up.
 

dai-leung

macrumors regular
Aug 21, 2017
150
39
Yes to both #1 and #2. TM has full admin rights to backup everything, including the OS.

You would just option key boot to the TM disk then erase the internal disk and restore. During the restore process you will get a screen like this. Just scroll down (back in time) until you get the OS version you want and restore.

Have some questions hope u have time to clarify.
1) I used time machine to back up Yosemite. Later the Mac was upgraded to High Sierra. Does this mean that I can continue to use the same time machine HD to do time machine backups on Sierra without doing something special?

2) if some goes wrong, I could use “option key boot to time machine” then select time machine restore to restore to Yosemite?

3) further, I can upgrade to Mojave and can restore using time machine backups to either Yosemite or High Sierra?

4) Using this approach, there is no limit on the number of OS upgrades that can be stored on the time machine backup drive? If the Mac is 10 years old, one can go back to the original OS on the Mac (when the Mac was purchased) from the time machine backups?
 

KALLT

macrumors 603
Sep 23, 2008
5,333
3,363
All can be answered with: yes. If you have snapshots of every previous version of macOS, then you should be able to revert to any snapshot you have, all the way back to the first snapshot. The Time Machine history is preserved when doing this and new snapshots are added to it, if you restore to a snapshot.

The only limit is the size of the Time Machine disk itself. If you want to keep at least one snapshot per OS version, you can delete individual snapshots by using the Time Machine app.
 

HDFan

Contributor
Jun 30, 2007
4,878
1,861
You would just option key boot to the TM disk then erase the internal disk and restore.

Make sure that you have multiple backups (best one not TM) before you do the erase in case the TM restore fails.
 
Last edited:

Weaselboy

Moderator
Staff member
Jan 23, 2005
33,183
13,621
California
Have some questions hope u have time to clarify.

Yes to everything you said assuming you have a large enough hard drive to hold data going back that far. If you option key boot to the TM drive and select restore you will get a screen similar to this one where you can pick what date/OS version you want to restore from.

The only issue is say you installed High Sierra Oct. 1, then Dec. 1 decide to restore back to Yosemite... that will work fine, but the restore would not include any changes or data from between those two dates since you are restoring to a point back in time at Oct. 1.

iu.jpeg
 

dai-leung

macrumors regular
Aug 21, 2017
150
39
Thank you all for answering my questions. I am surprise that this seemingly very useful time machine feature is not well documented at least for a non computer person like me. I am glad that you all make it clear how to use it with confidence.

I have more questions, hope you all have time to further clarify so that non computer persons could use this feature with more understanding.

The only limit is the size of the Time Machine disk itself. If you want to keep at least one snapshot per OS version, you can delete individual snapshots by using the Time Machine app.
What is this “Time Machine app”? Where can I get this app? Can I use this app to delete all backups except the last backup before each upgrade to a new OS?

Make sure that you have multiple backups (best one not TM) before you do the erase in case the TM restore fails.
That will be much safer if there is an easy way to do what you suggested.
Through MacRumors forums, I learned of CCC and is currently also using CCC to backup. But to do this multi OS backups, I would need separate drives to store the last backup of each OS version. I had also thought of having multiple partitions. But my difficulty is not knowing how to anticipate the size of the partitions as well as to anticipate how many partitions I would need. Just wondering if you know of a better way to accomplish the same goal.

The only issue is say you installed High Sierra Oct. 1, then Dec. 1 decide to restore back to Yosemite... that will work fine, but the restore would not include any changes or data from between those two dates since you are restoring to a point back in time at Oct. 1.

View attachment 877863
Thanks, you always give good advices and sharing your knowledge!

But I am confused, can I not use the backup of Nov 30th, that is the last TM backup right before the Dec 1 upgrade? Better yet, after upgrade follow KALLT’s approach and delete non essential backups to save space?

Also not clear about the backup disk size requirement. Assume that I have 1TB of data before upgrade and the size of the new OS is 1GB after installed. After install, I do a new time machine backup. The size of the TM backups is 2TB+1GB or 1TB+1GB?

I learned that time machine uses links between successive backups. Assume I have been using this store all OS upgrades approach for the last 10 years, is there a non zero probability that one of the links get corrupted and as a result, all successive files linked by that link will be corrupted as well? Is there a way to test for corruptions or to test the health of TM backups?
 

Weaselboy

Moderator
Staff member
Jan 23, 2005
33,183
13,621
California
But I am confused, can I not use the backup of Nov 30th, that is the last TM backup right before the Dec 1 upgrade? Better yet, after upgrade follow KALLT’s approach and delete non essential backups to save space?

Yes... I just made up the dates as an example. Whenever the last backup was on the old OS version can be used as a restore point to get back to that OS version.

I never suggest messing around deleting things in a TM backup. If you delete the wrong thing you can break the entire backup.

Also not clear about the backup disk size requirement. Assume that I have 1TB of data before upgrade and the size of the new OS is 1GB after installed. After install, I do a new time machine backup. The size of the TM backups is 2TB+1GB or 1TB+1GB?

If you update the OS on a drive that is using 1TB of space, then TM backup will not backup the whole 1TB again. It will only backup what has changed as a result of the OS upgrade. So there might only be 20-30GB of changes to backup.

I learned that time machine uses links between successive backups. Assume I have been using this store all OS upgrades approach for the last 10 years, is there a non zero probability that one of the links get corrupted and as a result, all successive files linked by that link will be corrupted as well? Is there a way to test for corruptions or to test the health of TM backups?

You can run Disk Utility First Aid on the TM disk to make sure it is okay.
 

dai-leung

macrumors regular
Aug 21, 2017
150
39
Hope I have not exhausted my welcome by asking more questions and hopefully your new answers will also benefit other future readers of this track.

I never suggest messing around deleting things in a TM backup. If you delete the wrong thing you can break the entire backup.
The is serious. Any idea what is this wrong thing?

If you update the OS on a drive that is using 1TB of space, then TM backup will not backup the whole 1TB again. It will only backup what has changed as a result of the OS upgrade. So there might only be 20-30GB of changes to backup.
Very welcoming news!

You can run Disk Utility First Aid on the TM disk to make sure it is okay.
This will ensure there is no corruption occurred in the entire TB disk and the TM backups are error free?
 

Weaselboy

Moderator
Staff member
Jan 23, 2005
33,183
13,621
California
Any idea what is this wrong thing?
I'm talking about system files the OS needs to run. As cheap as large disks are, I just think it is better to buy the size disk you need and not manually mess around inside the TM backup.

This will ensure there is no corruption occurred in the entire TB disk and the TM backups are error free?

Well, that is the idea anyway. I guess there are never any guarantees. That is why many people keep multiple backup disks.
 

KALLT

macrumors 603
Sep 23, 2008
5,333
3,363
What is this “Time Machine app”? Where can I get this app? Can I use this app to delete all backups except the last backup before each upgrade to a new OS?

The app is located in the applications directory.

The is serious. Any idea what is this wrong thing?

You can use the Time Machine app to view and manage your snapshots. You can navigate to a snapshot by using the arrow buttons or the timeline to the right. You can delete a snapshot by right-clicking on a folder, file or the window itself and selecting “delete backup” (it will ask for confirmation). Time Machine will then safely remove the snapshot, leaving all other snapshots intact. Alternatively, if you are comfortable with command-line tools, there is tmutil.

Thanks! Would you recommend using “First Aid on the TM disk” once a month?

Not needed. Time Machine will frequently do file-system consistency checks in the background and notify you if something is wrong with the volume.
 

dai-leung

macrumors regular
Aug 21, 2017
150
39
@KALLT
Thank you for your further clarification on the issue of TM deletion and on first aid! The app that you mentioned is it called “time machine” or some other name?
 

dai-leung

macrumors regular
Aug 21, 2017
150
39
Very confused about CCC’s snapshots. If you could help me to understand it, I would appreciate it.

To seek your help, I have constructed the following example to test my understanding to see if correct.

My understanding is that an APFS’s Snapshot is a picture of the Mac’s current state of the file structure. It also put a lock on each and every files preventing each from being modified. A file is free to be modified again when the corresponding snapshot is deleted provided there is no other snapshot controlling the same file. With this property, a snapshot can recreate the state of the Mac to the state when the snapshot was taken.

Also In APFS, changes made on a file (delta file df) is stored at a different location from the original location. These two files are separate files.

My CCC backup drive as well as the Macintosh HD are both SSD. In CCC, I enabled snapshot support for both.

On day 0(D0), the new Mac has Yosemite (call it state S0) then I do the first CCC backup so on my backup disk it has S0. When the backup is completed, CCC takes a snapshot of the destination drive, called it destination-snapshot(D0). It prevents S0 from being modified. Future CCC backups are not allowed to modify any file in S0.

On day 1, I make changes to some files. Lumping all these changes together and call it dS1. The mac’s state is represented by S1=S0+dS1. Also time machine makes 10 snapshots on the Macintosh HD, called them source-snapshot1(D1),...., source-snapshot10(D1). I then do a CCC backup. The CCC destination-snapshot(D1) should consist of S1 (=S0+dS1) and snapshots created on the source by TM, source-snapshot1(D1),...., source-snapshot10(D1). This destination snapshot prevents all of them from being changed.

On day 2, I upgrade Mac to high Sierra and S2=S1+dS2 where dS2 represents the new system files of High Sierra. Just like in the time machine case as pointed out by weaselboy, CCC does not know that the Mac OS has changed, it only know some files have been changed and dutifully records the changes like before. So the CCC destination snapshot on day1 allow me to restore the Mac to the state on day1, as well as to any state represented by the snapshots on the source: source-snapshot1(D1),...., source-snapshot10(D1). And likewise , the CCC destination snapshot(D0) allows me to restore to the state S0 on day 0.

On day 3, I upgrade the Mac to Mojave. Following the same reasoning, from the CCC destination snapshots on day 0, day 1 and day 2, I should be able to restore the Mac to Yosemite or HIGH Sierra, just like time machine would.

With these approach, all the past OS upgrades could be restored from the CCC backups. Am I correct or I am totally wrong?
 
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