Multiple Macs doing Time Machine backups to the same external drive? What happens when it fills up?

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Multiple Macs doing Time Machine backups to the same external drive? What happens when it fills up?

If you are backing up two computers using Time Machine to a single external drive, what happens when the drive fills up? The way Time Machine works, as I understand it, is that when a single computer is doing a backup of, say, 1 TB of data on that computer's drive, to a 3 TB external drive, all of the current versions of files are kept, and all previous versions of files are also kept until the disk drive fills up, at which point Time Machine starts pruning and discarding the oldest files to keep the external drive's capacity being exceeded.

But what if two 1 TB computers are backing up? When computer #1's Time Machine backup is the first one responsible for filling up the 3 TB external drive with all its prior versions of files, what happens to the older version of files with computer #2? I'm assuming that computer #1's old files are pruned to keep it under the limit, and then when running Time Machine for computer #2 it will immediately start harshly pruning away older files because the external drive is already full?

It's on my mind because the Western Digital professional NAS devices running their new operating system can no longer have multiple Time Machine Backup Shares, each with their own capacity limit, receiving just one computer's Time Machine backups for its own computer. Now you have one single Share designated as a master Time Machine Backup Share with a designated maximum storage at least equal to the capacities of the computers which will all back up to that one share. I guess this new way isn't much different than a single storage device with its own hardware capacity as opposed to the logical capacity of a "Share" on a NAS device, but I want to know what happens.

I personally use Carbon Copy Cloner to do backups of critical items such as movies and photos.

MacBook Pro 15″, macOS 10.12

Posted on Apr 8, 2022 11:58 AM

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Answer 1

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ku4hx

Level 7

23,586 points

Apr 8, 2022 12:11 PM in response to mvgossman

Every Mac should have a dedicated HDD; better yet two. That's the best practice; do you really want to lose 4 mac's backups all at once if the one drive fails? They do fail, even new ones..

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Answer 2

Apr 8, 2022 12:21 PM in response to mvgossman

Time Machine in Monterey wants an APFS formatted drive, and unlike the prior HFS+ formatted drives on older operating systems, there is no topmost backups.backupd folder where the individual hostnames and their backup hierarchy are kept apart.

In APFS drives, all backups are date/time stamped as a folder with the contents bearing the current backup hierarchy. There is no means to separate multiple machine backups on the Time Machine drive, and you will simply end up with twice as many date/time stamp folders with no way to know which Mac is represented by any folder.

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Answer 3

mvgossman Author

Level 1

15 points

Apr 8, 2022 3:51 PM in response to ku4hx

That's a fact, but what I'm working on is a PR4100 Western Digital NAS 4-bay drive with RAID-5 that is one part of my backup plan. The intent is for it to act as the automatic and continual Mac backup method, via Time Machine, with other independent USB drives including offsite storage as secondary backups. So I'm comfortable using the NAS in this situation, but I'm totally confused, and Western Digital is confused, as to what happens when Time Machine backs up two machines to one external disk. Taken to an absurdity, if you have a 4 TB external drive and computer #1 has built up 3.9 T of backed up data, and computer #2 enters with another backup of its own of say 1 TB, what happens? Is Time Machine on computer #2 empowered to pull older file versions from computer #1's TM backup in order to make room for computer #2's own files? Pretty important question for sure.

But this is a heck of an idea, just get another NAS drive dedicated to backing up computer #2. Brute force, just add money!

I changed HE double toothpicks to "heck", didn't know how puritan this forum could be.

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Answer 4

Barney-15E

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Apr 8, 2022 7:22 PM in response to mvgossman

A NAS is not an external disk. It is a network file server. As such, Time Machine will make individual Sparse Bundle Disk Images for each computer backing up to the NAS. When there is no longer any space on the NAS, each backup will be handled individually by Time Machine on each computer. Neither Mac will know about the other Mac's Time Machine backup.

If it is some hybrid network device that is mounted like an external drive, then Time Machine will only allow one Mac to backup to that device. You would have to somehow configure it to share out partitions (or partition it in Disk Utility, if possible).

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Answer 5

mvgossman Author

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Apr 8, 2022 10:30 PM in response to Barney-15E

The NAS is a network file server, but externally the shares appear to Mac OS as an external drive. The unique thing is that when you create a share and designate it as a Time Machine share, the Mac TM application recognizes it as an external drive suitable for using as a "drive" for backups. It used to be possible to create multiple shares as TM shares, but no longer. Western Digital is very vague about how it works, but claims that if you have two computers using TM to that share, it will work.

It probably works the same as when you have an external drive attached to computer #1, and computer #2 is configured to also backup to that drive attached to computer #1. It is possible to use TM to back up to another computer's external drive by setting up sharing permissions. So then the question becomes, when computer #1 fills up the drive, what happens when computer #2 tries to muscle in when there's no more storage?

Unless I have some assurances that it works, I don't see any alternative but to use an external USB drive for other computers or have an NAS designated to each one.

I have three USB drives attached to my main computer, two use Time Machine, and one uses Carbon Copy Cloner to backup the most critical things. I also use Carbon Copy Cloner to backup to a portable external drive that I use to copy to a NAS at a second home. That's how paranoid I am about safeguarding things.

The nice thing about Time Machine is that it keeps older versions of files, and deleted files, for a long time, and are accessible very simply.

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Multiple Macs doing Time Machine backups to the same external drive? What happens when it fills up?

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