More than 1 physical HD for a single Time Machine backup?

[Rico Muerte]

Can 2 or more physical drives be used for a "single" time machine back up?

Ive got a 2012/5,1 cMP with 4 physical drives - 1 for the OS/apps, 1 for general/personal files, 1 for all music recording, & 1 for time machine.
My time machine drive (4TB) is nearly full & could not complete the latest back up. I've got a few smaller drives around & wanted to know if I could use one in addition to the current TM drive for back ups.
Once again, 2 different physical drives for 1 TM backup...?

Thanks

 

[Bigwaff]

Not really. Each drive will be an independent backup drive. Post the exact error message you get when TM fails. TM will purge the oldest backups to make room for new ones. What version of macOS on cMP? Is your TM drive formatted HFS+ or APFS? Your TM drive should be at least 2x the storage size of the combined drives being backed up, if not 3x/4x.

 

[Fishrrman]

OP wrote:
"Ive got a 2012/5,1 cMP with 4 physical drives - 1 for the OS/apps, 1 for general/personal files, 1 for all music recording..."

Interesting.
I have a 2024 Mini, with the internal drive "hard partitioned" 4 ways:
- boot
- main
- media
- music

I DO NOT use time machine, however.
Instead I use CarbonCopyCloner to back up the 4 volumes.
My main backup is an external SSD, again partitioned into 4 volumes.
I also maintain backups (again, by CCC) on other drives in other locations.

Backups created with CCC do not forever "grow" in size (eating up disk space) as do backups in tm. However, CCC also offers something called the "safety net" which DOES archive older versions of changed files (it's enabled by default but you can turn it off, which is what I do).

There is also SuperDuper, which is a little "less-featured" than is CCC, but still does basically the same thing.

 

[benwiggy]

Once again, 2 different physical drives for 1 TM backup...?

Each individual TM snapshot essentially says "these files are different, and the rest are like these other ones already on the disk".

So it can't point to other files on a separate device.

My time machine drive (4TB) is nearly full & could not complete the latest back up.

How much data is on the other 3 drives? Ideally, a TM volume needs to be at least 2 or 3 times the size of the data you want backed up. (I use 4 TB drives per 1 TB of data.)

Usually, TM should start thinning older versions to recover space for the latest version, but if there's too much data to do this, then you might see errors.

I would simply put the current TM drive in a cupboard as an archive, and start from scratch on a new one.

 

[Rico Muerte]

Each individual TM snapshot essentially says "these files are different, and the rest are like these other ones already on the disk".

So it can't point to other files on a separate device.



How much data is on the other 3 drives? Ideally, a TM volume needs to be at least 2 or 3 times the size of the data you want backed up. (I use 4 TB drives per 1 TB of data.)

Usually, TM should start thinning older versions to recover space for the latest version, but if there's too much data to do this, then you might see errors.

I would simply put the current TM drive in a cupboard as an archive, and start from scratch on a new one.

Thanks - the issue is I dont have the money to buy a bigger drive at the moment & had a couple of 1TB drives laying around unused...

 

[ignatius345]

Backups created with CCC do not forever "grow" in size (eating up disk space) as do backups in tm.

This is false, unless you're talking about setting it up to just make a 1:1 copy of a drive and never save any revised/changed files. Not a great way to back up, IMO.

By default CCC takes snapshots of your source drive each time you back up. Each backup saves the files that have changed. You end up with multiple versions of changed files (a huge part of why we back up in the first place), and those versions do of course take up space on the backup drive. Over time, CCC purges the oldest backups to make room for the newest ones -- quite similar to Time Machine's behavior, in fact. These days CCC works a lot more like TM than it used to.

However, CCC also offers something called the "safety net" which DOES archive older versions of changed files (it's enabled by default but you can turn it off, which is what I do).

Safety Net is deprecated. CCC now uses APFS snapshots on modern systems, as I described above. Here's what they say in their documentation:

The topics in this article are not relevant to APFS-formatted destination volumes that have CCC snapshot support enabled. For those volumes, CCC leverages snapshots to implement the SafetyNet functionality, and those snapshots aren't affected by any of the shortcomings described here. Also, bear in mind that SafetyNet is a safety mechanism; it is not designed to be part of the backup strategy. We recommend using the destination exclusively for one backup task and disabling SafetyNet.

OP: This is all a big detour from your original query about Time Machine, but it is worth pointing out that CCC is quite a bit more flexible than TM and might give you some options for spreading your backups out among several 1 TB drives in addition to your 4 TB drive. Time Machine is fine, IMO, but in typical Apple fashion it does what it does and you can't do much to customize its behavior.

 

[ignatius345]

My time machine drive (4TB) is nearly full & could not complete the latest back up.

Are you saying the size of your total sources has grown larger than 4 TB?