Time Machine backup ssd apfs

My wife has an M1 iMac. I set up two backup drives for her; one is an SSD, the other is a mechanical drive (HDD).

For the first complete backup, the SSD took about 15 minutes, and the HDD took about 50 minutes. She is using about 300 GB. Subsequent backups with either drive (Time Machine alternates when one has two backup drives) take seconds. The mechanical one is no doubt slower but either one is extremely fast. Both are APFS (they have to be for Time Machine).

I think where one might really notice the slowness of the mechanical drive is if it were to be a boot drive, versus an SSD as a boot drive.

Even if the mechanical drive is noticeably slower under APFS, Time Machine runs in the background and one might not even notice.

Personally, I prefer SSDs and have been switching slowly to use more and more SSDs and fewer HDDs. That is not just due to the speed, but rather their ruggedness and lack of moving parts, which I feel may make them more reliable in the long run. They are also smaller and more portable.

denydean wrote:

I have a M1 iMac. Time Machine now supports APFS. I've been using a HDD. Should I be using a SSD? APFS is optimized for a SSD. I heard HDD can slow down when formatted with APFS.

The reason for the slowdown won’t occur on a backup. It has to do with the breaking up of a file when editing. When you rewrite to disk, the pieces become scattered all over the drive. Old file systems tried to control that so that the drive head didn’t have to bounce all over the place. None of that is a problem for SSD’s.

A Time Machine backup doesn’t edit any of the files, so they are pretty much written to contiguous storage.

HDD’s are slower in general, but for a backup, it’s not much of an issue.