Time Machine Backup disk size vs back up size vs restoration

Question

Time Machine Backup disk size vs back up size vs restoration

Couple of months ago I faced issues with my MacBook and during this I had to do lot of backup and restoration kind of stuff to keep me going. One thing which is not clear to me is the right size of backup disk and issue it could create during restoration if proper disk size is not chosen.

Assume that

A) Installed hard disk inside macbook is 1 TB.

B) The actual used space is say 200 GB

C) The Time Machine backup disk is 500 GB

If I take a backup of A) on C) then clearly backup process will not have any issue because 200 GB is what is really needed. Over the time also if usage grows but remain less than 500 GB the C) will still accommodate the backup of A)

Let's say at a point in time, the actual usage has grown to total 350 GB and I have a latest backup in C) and A) crashes for some reason and I installed a alternative disk of the total capacity 500 GB in my MacBook.

Will I be able to restore from C) on to the alternative disk whose capacity if compared to A) is half but still more than actual space required.

I faced this issue when I tried to restore from Recovery mode -> Time machine backup hence asking. The recovery process simply refused to install on smaller hard disk.

It is correct observation that in order to restore the backup the destination disk should have equal or greater capacity compared to the disk whose backup we are going to restore. In other words, the actual space used on disk is immaterial? Only source and destination disk capacity matters.

MacBook Pro, macOS Sierra (10.12.5), 1 TB SSD, 4 GB RAM, Mid 2010 Model

Posted on Jun 13, 2017 11:04 PM

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Answer 1

tjk

Level 9

51,192 points

Jun 14, 2017 6:32 AM in response to Ishwar Jindal

Hi,

" Time machine backup hence asking."

Asking what?

"It is correct observation that in order to restore the backup the destination disk should have equal or greater capacity compared to the disk whose backup we are going to restore. In other words, the actual space used on disk is immaterial? Only source and destination disk capacity matters."

That question would be moot, as you said the replacement is 500GBs and the TM backup disk is 500GBs. In any case, it's the size of the data used on the individual TM backup that matters.

Did you follow the instructions from Apple? Use Time Machine to back up or restore your Mac - Apple Support

Restore everything:

To restore everything in your Time Machine backup, use macOS Recovery.

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Answer 2

Jun 14, 2017 7:28 AM in response to tjk

What I am asking is that if HDD which was backed is 1 TB, space used on that HDD is only 350 GB (hence time machine backup size is 350 GB) and destination HDD (where I am trying to restore) is 500 GB then is macOS recovery supposed to install the backup or not? I believe you are saying No.

If no then wondering if there any configuration/settings I am missing which can allow a complete restore via recovery even if disk is smaller. Ideally when my actual usage is only 350 GB then why recovery need 1 TB disk when in reality 500 GB can also accommodate??

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Answer 3

Jun 14, 2017 9:02 AM in response to Ishwar Jindal

What is stored on the Time Machine drive is not ONE Backup, but a multitude of backups from different dates & times. The size of the Time Machine drive is large so that you can have more different Backups available without immediately having to throw stuff away to make the next backup.

The total size of the Time Machine drive has NOTHING to do with being able to Restore. There are many factors which could have precluded being able to Restore, such as:

• If the drive was not GUID partition Map with a MacOS HFS+ Volume.

• If you were trying to restore a backup including MacOS onto the active Boot Drive.

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Answer 4

Jun 14, 2017 9:33 AM in response to Grant Bennet-Alder

Thanks Grant for the reply. I know backup is not single file but rather a layer of multiple backup with information about every point in time when backup was taken.

Below was the state when I faced issue.

My time machine backup disk has always been Seagate 500 GB disk.

I originally had a Toshiba 500GB HDD installed in my MacBook and I used to take its backup on Seagate disk. That toshiba disk had OS from Leopard till most versions of El Capitan.

Last year I upgraded to Samsung 1TB SSD but kept my backup disk same I.e. Seagate because all along my actual disk usage never crossed 500GB. It remained around approx 300 GB even on Samsung SSD. So Seagate backup disk was fine.

Some time in January this year I installed an macOS Sierra update without first taking a backup of my SSD. My last backup on Seagate was around in Nov 2016. The macOS update prevented me to login into my MacBook.

At that time (I mean in Jan) I had the option of either

1) restoring Nov 16 backup on Samsung SSD but that would mean loosing Nov 16 to Jan 17 worth of data

or

2) wait for the next update of macOS (hoping apple would fix meantime) but to keep going install Nov 16 backup on an alternative disk.

I opted option 2 and installed a blank 500 GB disk. On this disk when I tried to install Nov 16 backup from Seagate disk (which was taken from 1 TB SSD, macOS) via MacOS recovery the option was greyed out. I mean I choose the Nov 16 backup and destination as 500 GB disk but continue button was disabled.

Luckily on the same Seagate backup disk, I had El Capitan backup from Jun 2016 which I took when I was originally on 500 GB disk (I.e. Prior to SSD upgrade) and that Jun 2016 got successfully restored on blank 500 GB.

I failed to understand why Nov 16 backup didn't work but Jun 16 worked. The only logical conclusion I could draw was because of the diff between 1TB vs 500 GB. I mean size matched it worked but when size was smaller then it didn't worked. All along my actual disk usage is less than 350 GB.

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Answer 5

tjk

Level 9

51,192 points

Jun 14, 2017 7:31 AM in response to Ishwar Jindal

"In any case, it's the size of the data used on the individual TM backup that matters." So my answer is "yes," 350GBs fits on a 500GB HDD.

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