Very slow Time Machine Backup to new disk Sonoma Mac Mini M2

Note Leroy's comment, Time Machine works in the background, when the machine is active. If you are working the processor and/or disk hard while doing a Time Machine backup, the backup will slow considerably.

I recently did a brand new Time Machine backup on a MacBook Pro 2019 16-inch running 14.4.1 and it did 400 GB in about 3 hours. The computer had nothing else running and its internal drive was 60% free. This was used to migrate to a new M3 MacBook Pro and its first Time Machine backup, also on an idle machine with plenty of internal disk space empty, took about 3 hours (400 GB).

So yes, your TM backup should be well under 12 hours.

But 14.4.1 does fine with these backups, as my measurements show, even with an older Intel 2019 computer.

(1) If your computer is very busy the TM backup slows.

(2) If your internal drive is close to full that also will slow down all activity in the Mac. For instance, you mentioned 800GB, that would be plenty of free space on a 1 TB drive, but if you are doing something like running Lightroom on a large catalog, not only will that slow down the backup considerably, software like that can use hundreds of GB of scratch space, which means temporarily at least the internal drive is very full and its paging and other processes slow considerably. You haven't provided these details.

(3) The external drive and its cable are the other factor, as Leroy points out. You can try a new cable and also a new backup drive. It's good to have multiple TM backups, actually, anyway. You can also check the condition of the drive with DriveDX and its performance with BlackMagic. Very slow Time Machine backups are one possible symptom of a degraded backup drive.

(4) The projected time remaining for the TM backup can be far off and change quickly. But by now yours has finished, no doubt, anyway.

gtrnstuff1 wrote:

While this was going on, the Mac was very sluggish. Cursor was jumping unpredictably while trying to work.

So after 8 hours, showing 10% progress, I ejected the drive and unplugged it.

I took care of some tasks I needed to do on that Mac, did a normal, incremental TM backup to my other drive that was working.

I plugged in the drive that had been problematic, it mounted, and everything worked smoothly.

The full backup I had started finished in about 1.5 hours.

Computers. Go figure.

Good computing gtrnstuff1 !

So it is a non-issue, or a drive specific issue...

If the drive proves to be problematic replace it for one that works.

gtrnstuff1 wrote:

I used to be able to do a fresh TM backup of about 800G in much less than 12 hours.

At the rate this one is going, it could be 24 or 36.

I see lots of threads about slow backup with various OS and hardware.

Is there anything a user can do?

Or just wait for Apple to fix it in a future OS update?

Currently Sonoma 14.4.1

Mac Mini M2

Newly formatted APFS case sensitive 2TB drive

Newly formatted, OK. You do not say if this is a brand new drive or an old drive— not much to go on here

So you are making a new full database backup...

Report back in couple weeks if this sorts itself out. TM works in the back ground, when the machine is active.

ref: Use Time Machine to back up - Apple Support