If you’re intending to rely on Time Machine backups in macOS 10.15.3, you might want to use an alternative as well. I can now confirm that making the first full backup using Time Machine can take so long that it may not be feasible, and that some users are also experiencing failure to restore from an existing backup.

I have already given considerable detail, including log data, on the problem with making first full backups. Since then, I have looked at different scenarios and confirmed that this isn’t related to any underlying disk or file system problem, nor is it the result of using SoftRAID. As I’ll explain in future articles, I have completely re-initialised the four 2 TB SSDs in my ThunderBay 4 (Thunderbolt 3) enclosure. Each is now a standalone SSD, three formatted in APFS and the fourth in HFS+, using Disk Utility in macOS 10.15.3. Once formatted, each has passed Disk Utility’s First Aid checks without any errors, as have the source volumes used to make these backups.

When Time Machine is enabled, the first full backup proceeds very briskly until 10-20% of the source has been backed up. Progress then suddenly slows to a crawl, taking an excessively long time to complete, when you have the patience to let it continue.

A good example is the only successful first backup which I have been able to complete in 10.15.3 on this iMac Pro. I set Time Machine’s exclude list to ensure that all mounted sources were not backed up, apart from a single folder containing a total of 1.93 GB in just over 10,000 files. The destination for the backup was a freshly-formatted Samsung 860 EVO 2 TB SSD with a SATA 3.2 interface, which consistently achieves better than 500 MB/s read and write transfer rates when tested using Blackmagic Disk Speed Test.

When Time Machine performed that first backup, it took 98 minutes to complete, for almost all that time transferring data so slowly that progress was almost imperceptible.

According to the log, that first backup started at 12:44:18.6, and copying had started by 12:56:45. Reported transfer rates during backup varied between 0.0-1.29 MB/s, and the backup was complete at 14:22:36, an elapsed transfer time of 01:25:51, just short of 86 minutes, at an overall transfer rate of 0.37 MB/s according to the data in the log.

The same SSD, and three other identical units in the same ThunderBay 4 (Thunderbolt 3) enclosure, deliver far superior performance when used with current releases of Carbon Copy Cloner and ChronoSync, which both back up the same folder to either HFS+ or APFS without any errors.

I have been unable to reproduce this on a MacBook Pro 16-inch 2019 with 10.15.3, backing up to another freshly-formatted external SSD. However, its first full backup of about 83 GB took 44 minutes in all. According to log entries, copying took place at an average rate of 37 MB/s, one hundred times faster than on the iMac Pro. At that transfer rate, performing a first full backup of 1 TB would take 450 minutes, or 7.5 hours.

Once this first full backup has completed, if that is feasible, second and subsequent backups appear both brisk and brief. This suggests that the problems lie in Time Machine’s first backup strategy.

There are indications that one potential cause of extremely slow first backups is a large Revisions/Versions database stored on the volume being backed up, in the locked and inaccessible folder .DocumentRevisions-V100 at the root of the volume. One potential workaround might therefore be to add that folder to Time Machine’s exclude list, for each writable volume to be backed up, although that would result in the loss of saved version data for all documents on that volume. One of the advantages of using Time Machine for backups is it preservation of document versions.

Several users have reported to me that they too have experienced serious problems with Time Machine in 10.15.3, both in making first full backups and in trying to restore from existing backups. At least one of these has been reported to Apple as a bug in Time Machine, and has apparently joined several previous reports of the same problem.

If you have existing backups made using Time Machine and have updated to 10.15.3, I recommend that you use another independent backup tool to ensure that you can restore important files if you were to encounter problems using Time Machine to do so. This is best practice anyway, not to be totally reliant on any one backup tool. I recommend, from personal experience and users’ comments, Carbon Copy Cloner, SuperDuper!, and ChronoSync as being up to the task. Other backup apps may be just as good, of course, but I don’t have personal experience of them.

If you are intending to make a full first backup using Time Machine in 10.15.3, you should be wary of its potential to never complete, and be prepared to fall back to a substitute backup system. If you do encounter very slow progress during the first backup, cancel it, remove the incomplete backup, add the .DocumentRevisions-V100 folder at the root of each volume to be backed up to Time Machine’s exclude list (press Command-Shift-. to see hidden items in the file selector dialog), then try again. Please let me know if that mitigates the problem.

I am very grateful to the many who have provided me with details of your experience, particularly James.