How to partition a drive for Time Machine

veehbJ wrote:

I wanted to use a 5 TByte drive with Time Machine.

I would simply erase/reformat/initialize as new and start over with your full 5TB.

GUID/APFS encrypted

Erase and reformat a storage device in Disk Utility on Mac

Erase and reformat a storage device in Disk Utility on Mac - Apple Support

TM likes to have its own dedicated drive.

veehbJ wrote:

I've read that you should not share data storage on a disk that Time Machine uses. Do you see a problem with using this same drive (after reformatting) for data storage along with the Time Machine file?

To expand after I re-read your question, as I mentioned, it's technically possible with two volumes on the drive. But can doesn't always mean should. There are two types of drives, drives that have failed and drives that haven't failed yet. If the plan is to use one volume to back up your Mac and the other volume to store files that you don't want taking up the space on your internal drive, that's a risk. If your Mac fails, you have a backup. If the external drive fails, you have the Mac but maybe you don't have a backup for the non-backup volume on that drive.

A common strategy is 3-2-1, meaning three copies of your data, two of which are backups and one of which is stored offsite.

In my case, my primary backups are to a 10 TB NAS is configured with 6 volumes, 5 for Time Machine backups of 5 Macs and one for general file storage. But I keep secondary TM backups of all the Macs on a pair of 4 TB SSDs that I swap offsite every week, and I have another set of SSDs that I sync with the general file storage on the NAS. So for all my files I really have 5-4-1 for files on the Macs and 4-4-1 for files in general storage that aren't on a Mac (the NAS is RAID1 so everything on there is on two HDDs, plus the pairs of SSDs). Overkill? Perhaps. But if a Mac fails, it can easily be replaced and if the house burns down I still have all my data (at most losing 2 weeks worth).

You should dedicate a single drive to TM. It is not advisable to use the same drive for TM Backups and any kind of "live data" at the same time. That is not an effective backup strategy, and, based on my own observations, TM prefers to have a drive all to itself.

@neuroanatomist has described what is considered minimum best practice for backups .. 3-2-1 ... 3 copies - (1) your original data, (2) two backups, one of which is kept offsite.

IMHO you can never have too many backups. In my own case I keep 1 "everyday" backup plus 4 sets of periodic backups, all on independent drives -

1 "everyday" backup drive, always connected, with 2x daily automated backups of critical data.

2 monthly backup sets, alternating backups, one set updated each month

2 offsite backup sets, alternating backups, one set updated every 6 months

Full disclosure ... I don't use TM.

Apple recommends that you do not partition a disk, Time Machine notwithstanding. APFS containers convey the ability to add additional volumes to existing containers, which dynamically share space within that container. Time Machine will use all it can.

Partition a physical disk in Disk Utility on Mac - Apple Support <-- don't do that.

Add, delete, or erase APFS volumes in Disk Utility on Mac - Apple Support <-- do this instead.

Time Machine asked me if I wanted to partition the disk.

I'm not sure how you determined that information. It's not a good idea. It's your Mac though, and your data, so proceed accordingly.

John Galt wrote:

Time Machine asked me if I wanted to partition the disk.

I'm not sure how you determined that information. It's not a good idea. It's your Mac though, and your data, so proceed accordingly.

I suspect that is a non-technical reference to the option to limit the size of the TM backup that is available during setup.

That probably explains it.

Adding an additional APFS volume can be accomplished at any time. I still don't recommend it though. If a backup drive fails, the entire drive with all its volumes is probably useless (example). TM backup drives should be as plentiful as they are disposable.

Part of your strategy may depend upon the kind of drive you have. This starts getting technical but with some drives APFS may not be the best strategy. Time Machine will want to use APFS (assuming nobody uses anything but SSDs, but many multi-TB drives tend not to be SSDs) but you might be able to work around that and get it to use a format better suited to an older drive format. You basically set up a new drive with a few folders to make Time Machine think the drive already has a backup on it.

I don't going into a lot of detail here and will instead recommend some reading:

https://support.bombich.com/hc/en-us/articles/20686495048215-Choosing-a-backup-drive#recommendations

https://mjtsai.com/blog/2024/04/03/seven-years-of-apfs/#comment-4065459

https://eshop.macsales.com/blog/43043-using-apfs-on-hdds-and-why-you-might-not-want-to/#comment-257549

Time Machine didn’t actually “partition” your drive, it just created a 1 TB APFS volume inside the container and is treating that as the backup target. The rest of the space is still there, but TM will try to use whatever you give it.

If you want part of the disk for TM and part for general storage, the right way is to erase the drive in Disk Utility (GUID/APFS) and then add two separate volumes, one for TM, one for files.

Just keep in mind: if that single drive fails, you lose both your backup and your extra storage, which is why Apple (and most of us) recommend dedicating a drive just for Time Machine.

Just to be clear, there can be scenarios for using a partitioned drive. One reason is if you wish to use it for backing up using different utilities. I have two drives I use with my computer all the time. One is the internal boot drive and the other is an external HDD drive I use for "data". I back them up to a single large drive. I use Time Machine for one drive but I prefer using a different utility for the other. Since Time Machine does not like sharing a volume with anything else, I partition the backup drive. Since my backup drive is a HDD, I do not like to use APFS (see the links in my earlier post). I am just setting up this drive now but I plan on wiping it for my new computer, using HFS+ (trying the method I posted earlier), then having Time Machine back up my internal boot drive to one partition and have my external "data" drive back up to the other partition.

I use a dedicated 2 TB Crucial X9 SSD for Time Machine backups on my 1 TB M4 Mac Mini Pro. Keyword dedicated. I never use TM drives for anything beyond its TM purpose. If I need storage for other reasons, I purchase a separate drive for that purpose. I do not use the same TM drive across operating system upgrades.

Limnos wrote:

Just to be clear, there can be scenarios for using a partitioned drive. One reason is if you wish to use it for backing up using different utilities.

Indeed. I use partitioned SSDs to back up 5 Macs. Mea culpa for earlier referring to them as ‘volumes’. The issue with using multiple volumes to back up different Macs on an APFS drive is that as John Galt mentioned, the volumes are dynamically resized so one Mac could come to dominate the space. An alternative to partitions would be to use volumes but specify a limit for each Mac when setting up the TM backup.

I think a necessary starting point for all this is knowing what kind of drive this "5 TByte" drive is; a SSD or a HDD? If HDD, one is better off using using HFS+ (particularly if it is a lower cost SMR type) and not letting TM set it up. Then one is using fixed partitions.

My money is on the one in this case being a HDD. I haven't priced 5TB SSDs but I suspect they would be horriby expensive!

Reformat the drive (GUID, APFS), and create two volumes in Disk Utility, one of 1 TB for the backup, the other for file storage. Select the smaller one in TM Setup.