Adding a second external drive to Time Machine for full system backups on iMac

With your 21.5-inch iMac running macOS 15.7, you can use the two external drives for different backup purposes.

Keep your existing external drive as the Time Machine destination. 

macOS supports multiple Time Machine backup disks, so you could also add the unused drive to Time Machine. 

Time Machine rotates among the configured disks and, when a particular disk is used again, backs up the changes since that disk was last used. 

If you connect the second drive only periodically, its backup history will naturally be less current than the history on the continuously connected drive. (support.apple.com)

However, if your intention for the unused drive is to maintain a separate copy of your Mac's data and applications, rather than another Time Machine backup history, Carbon Copy Cloner (CCC) is another option. CCC can make an initial backup and subsequently update that backup with changes. 

You can run it manually when you connect the drive or schedule it. CCC 7 supports macOS 15 and Intel Macs. (bombich.com)

There is an important qualification with macOS 15: CCC's normal startup-disk backup should not be described as a traditional, bootable, byte-for-byte clone of the entire macOS installation. 

Apple's modern macOS system volume is handled differently from the Data volume, and CCC's standard backup focuses on the Data volume containing your applications, documents, settings, and other user data. (support.bombich.com)

Thus, the two-drive arrangement could be Time Machine on the existing drive for versioned backups and historical file recovery, and CCC on the unused drive for a separately maintained copy of your current data, applications, and settings. 

You could keep the CCC drive disconnected except when updating it.

If instead you want the unused drive specifically to be another Time Machine destination, you do not need CCC; simply add the drive to Time Machine. 

If you want an independent backup in addition to Time Machine, CCC provides a different backup method rather than merely duplicating Time Machine.

Apple's relevant documentation is Back up your Mac with Time Machine and Use Time Machine on your Mac to back up to multiple disks. CCC's documentation is available from Bombich Software.

A second Time Machine disk is not really a “full system copy” in the traditional cloning sense. On macOS 15, the startup volume is an APFS volume group with a sealed, cryptographically verified System volume and a separate Data volume, so copying “Macintosh HD” to another disk with Finder is neither a reliable system backup nor something you can simply restore as a bootable installation.

Time Machine is actually doing something more useful, it preserves a versioned backup of the user Data and system state, with APFS snapshots and metadata that Finder copy/paste does not reproduce correctly. If the second disk is going to be disconnected most of the time, I’d configure it as another Time Machine destination rather than periodically trying to clone the internal disk..... when you reconnect it, Time Machine can reconcile it from the point that disk was last backed up, while your always-connected disk continues its normal history.

If your real goal is protection against a failure of the primary Time Machine disk, though, keeping the second disk as an independently maintained backup is the more important part; using CCC for that can make sense, but think of it as a separate recovery/backup strategy, not a byte-for-byte clone of the sealed macOS installation.

Connect the drive, open Time Machine Settings, click the + button at the bottom of the dialogue box and add the drive.

Unless you've created a partition just for Time Machine then you'll lose anything on the drive.

It's preferable not to create a partition. It will work but Time Machine doesn't play well with others and it might cause problems.

You can trigger a TM backup when the disc is connected by opening TM settings, right-clicking on the disc and it will back up.

If you leave both discs connected TM will alternate, but both discs will always have a full backup.

You could, if you wished, use a Raspberry Pi to put your 2nd disc on your network (ethernet or WiFi) and it will be backed up to hourly or daily, depending on your prefs. If you want to do that then Google or search the Raspi forums - there's lots of help there.

Edit - sorry for the redundant post. When I opened it the replies weren't there and I didn't see Owls comprehensive response

"321" is generally recommended as a minimum method: Three backups, two methods, one off-site. I backup using Time Machine and CCC to different discs (NAS, Time Machine, USB HD). One disc is kept in the shed (in case the house burns down). I also have other less changeable files (e.g. older photos) backed up and stashed at my parents' house.

Again, thanks for the responses.  I have read them and am nearly certain that if I continue on this path I will add the second drive to Time Machine settings.

That said, I'm thinking of changing paths here.  I've read that it is recommended to keep 10-30% of the internal drive open.  I'm now much fuller than that and I am considering use of the empty drive is to move some of my user data from the internal to it.  The spare drive is a Time Capsule, so it is both old and a mechanical drive. I understand the risk of failure, but I have an external Time Machine drive and routinely copy my user data to another external drive stored off-site.

My understanding of networking is very weak and I'm quite unsure how to go about the connection. The iMac has a single ethernet port which is currently attached to my TP-link Deco XE75, as is my printer. I have a multi-port adapter which includes ethernet hanging from a USB-B port on the Mac. The Deco has an open ethernet port.

If anyone would like to chime in on whether or not this is a good idea, which data to move and how to make the connections I'd appreciate it.  My current internal drive usage looks like this:

I need to do a thorough cleanup before proceeding, but I don't expect those two largest item to change by much.  I share selected items from both Music and Photos to an iPad and an iPhone using an old-fashioned wired sync.  I do occasionally stream music from the iMac, but using one of the other devices in its place is not a hardship.  Documents that I use regularly, including some large spreadsheets, reside on my iCloud Drive.

[Edited by Moderator]

You should really try to maintain at least 100GB of free space on your 1T drive for smooth trouble free operation.

Free up storage space on Mac - Apple Support

To gain larger chunks of free space, you can move your Photos, Music or Movie Libraries to an external hard drive.

Move your Photos library to save space on your Mac - Apple Support

Change where your music files are stored on Mac - Apple Support

Move Logic Pro for Mac and MainStage content - Apple Support

Move your iMovie for Mac library - Apple Support

Move a Final Cut Pro library - Apple Support

In your case, it looks like moving either your Photos or Music Libraries to an external drive would give you plenty of free space.

The catch here is, that you may need two more external drives in addition to the current Time Machine backup drive.

A 1T or so external drive for your Photos and/or Music Libraries.

A 2T to 4T external drive to use as a second Time Machine backup drive.

<<  The spare drive is a Time Capsule. >>

Caution!

Apple File Protocol (AFP) is the only way to talk to a Time Capsule Drive inside a Time Capsule. AFP is deprecated, and will not get ongoing support.

The next version of MacOS 26 Tahoe will not let you start a NEW backup on a Time Capsule Drive. if you ERASE the drive you are done using it inside the Time Capsule.

... and the version after, MacOS 27 Golden Gate will not acknowledge that it exists.

The work-around when you get there is to remove the drive from the Time Capsule and place in an enclosure or 'toaster' style non-enclosure.

Thanks, but I’m not sure I understand your comments about additional drives. Time Machine is currently backing up to a 4T external drive. Are you suggesting the need for a second backup drive?

The current backup is handling the same amount of data. Could the backup settings not include both the internal drive & the external drive holding the Music library?

<< Could the backup settings not include both the internal drive & the external drive holding the Music library? >>

Time Machine by default automatically backs up every drive connected to your computer that is in a Mac-native format {APFS, MacOS extended HFS+ }.

Time Machine can not back up other format drives like exFAT or NTFS.

jack3030 wrote:

Thanks, but I’m not sure I understand your comments about additional drives. Time Machine is currently backing up to a 4T external drive. Are you suggesting the need for a second backup drive?

You were the one asking about adding a second drive for Time Machine.

The current backup is handling the same amount of data. Could the backup settings not include both the internal drive & the external drive holding the Music library?

If you follow those directions for moving your Photos or Music Libraries to the spare external drive. Then YES, it will be included in with your current Time Machine backups.

My apologies for letting this sit so long, but stuff happens. Thanks for the reply and I'll return to working on this as soon as time allows.

I was unaware of this. Thanks.