Getting best Time machine performance

[donawalt]

I have a second external disk drive on its way for another computer, but I first want to test its raw read/write performance against my primary MacBook Pro Time Machine backup disk. I have Blackmagic, but it won't test my existing external disk used by TM because it says it is read only. I don't want to mess up my existing TM backups on that drive, is there an easy way to compare performance of these two drives or do I have to bite the bullet and unselect that drive from TM using it? Thanks!

 

[chabig]

I wouldn't mess with the Time Machine drive at all. Forget about trying to "test" it. Time Machine won't max out the transfer rate on the disk anyway. But if you insist, make sure you have a second backup.

 

[w5jck]

I always schedule TM to do backups overnight so that it has plenty of time to do it without slowing down whatever else I’m doing on the computer. I use SSDs for extra storage space and all kinds of file copies except for TM. I find it is sufficient to use the much cheaper HDDs for TM, as long as I schedule TM backups during times I’m not using the computer. With HDDs you are looking at slower write speeds, perhaps 140 MB/s compared to 350+ MB/s with SSDs, but if the computer is idle during backup time then it really doesn't matter.

 

[donawalt]

OK, thanks @w5jck and @chabig good advice....

 

[w5jck]

BTW, as for TM, I find it useful to get a larger capacity HDD than needed to use. For example, my MacBook Air M1 has 1TB of storage built in. I bought a 4TB Toshiba HDD for $100 with tax. I don't need 4TB to backup 1TB, so I formatted the HDD as APFS (what TM uses), then partitioned it to use 2TB for TM and 2TB for all other storage. Even 2TB of TM is overkill for a 1TB computer, but better safe than sorry. This way I get the TM backups, but I also have 2TB of storage space to directly copy over files. So I get plenty of TM space plus quite a bit of extra storage space for files I don't need to access a lot. In other words, files I tend to archive but rarely access. Not bad for $100 investment. But as I said above, for files I store on a separate drive that I need access to more frequently, then I use the more expensive SSDs capable of 5-10 Gbits/s (which is 625-1250 MB/s).

 

[fivenotrump]

I usually budget 2½ times the size of the disk being backed up by TM. My nearly one year old iMac has a 1TB disk which is currently about two-thirds full. Time machine volumes are each using about 1.3TB at present. Of course much depends on usage patterns and how long you anticipate needing old backups for.

 

[donawalt]

You are right about needing more storage. I have one laptop that has a 4TB disk, and a 4TB backup/TM disk. Periodically it gets the "unable to complete backup" that has a complicated explanation that basically concludes with needing more space than the system disk.