Is it safe to upgrade to Catalina 10.15.5?

Now’s the time that those who’ve been cautious, and let the early adopters find all the bugs in Catalina, consider whether to upgrade. Is it safe to go ahead, in readiness for macOS 10.16, or are there still showstoppers which will make you regret it?

With Catalina, there can’t be a clear answer. Its two biggest problems for anyone upgrading haven’t changed, and will only get more challenging with 10.16: loss of 32-bit software support, and the security requirements imposed on executable code including notarization. If you’re still reliant on apps or any other software which uses 32-bit code, then you’ll only be able to run them in a Virtual Machine, or by dual-booting your Mac.

Notarization and related requirements shouldn’t stop simple apps, but if you need to use apps which load external code modules, like the high-end charting and analysis app Igor Pro, they don’t fit Apple’s new security model well, and many have either limited functionality or still won’t run properly. Some of these apps may require structural changes before they can be successfully notarized and fully compatible with Catalina.

Another serious issue for anyone coming from High Sierra or earlier is that Catalina must boot from an APFS disk. If your internal storage is a hard disk, that’s bad news unless you replace it with an SSD, or boot from a suitable external SSD. APFS is designed for SSDs, and performs poorly on hard disks. Careful analyses have shown that this isn’t something likely to go away either: if you want to continue booting from a hard disk, then you should stick at High Sierra or earlier.

Catalina has many warts and minor bugs, but there are three areas which have generated the most problems: the Mail app, Time Machine, and replacement apps for iTunes.


I don’t use Mail much myself, but Michael Tsai, who develops SpamSieve, has been tracking problems with Mail very carefully. He currently recommends that you should only upgrade to Catalina when you’ve made a complete backup, and if possible have archived all important messages, for example to his database EagleFiler. You should then ensure that you upgrade fully to 10.15.5 before opening the Mail app for the first time in Catalina. From then on, you should be careful to copy or archive all important messages – something which is good practice in any case.

My solution is much simpler: I use Postbox, and there are several other excellent replacements for Mail, including GyazMail, MailMate, Airmail, Outlook and Thunderbird.

Time Machine

Partly because of Catalina’s new boot Volume Group, which installs most of macOS on a protected System volume, and writeable files on a Data volume, and changes in the backup process, Time Machine users should be very careful when preparing to upgrade. If you possibly can, archive your current backups, including the last you make before upgrading, and start a fresh backup set with Catalina. Once you start making backups in Catalina, those and any older backups which are converted cannot be accessed by earlier versions of macOS. If all goes horribly wrong and you revert to Mojave (or earlier), you’ll want your old backups back. If you’ve let Catalina convert them, then they are lost.

Don’t start Time Machine backups immediately your Mac has started up in Catalina, as you may well encounter a bug which makes the first full backup, and some subsequent backups, take a great many hours or days. This appears to be the result of Time Machine choking on the hidden versions database in the .DocumentRevisions-V100 folder at the root level of each writeable volume, particularly the boot Data volume. Add that to the exclude list in the Time Machine pane before turning automatic backups on.



If you run into problems with Time Machine, download and install my free T2M2 utility and read its Help file: this details how you can use it to diagnose choking during backing up, and problems between automatic backups.

Music and other replacements for iTunes

If you’re a heavy iTunes user, be aware that in splitting that app up, its replacement components are far from mature and feature-complete. The Music app in particular doesn’t have the same features, and some users experience problems with cover art, playlists, and converting their iTunes library. Although you may want to keep old audiobooks in iTunes’ successor Music, they’re now intended to go into the Books app, where support is still primitive.

Boot disk

You should already be aware that Catalina boots from an APFS Volume Group consisting of a volume named Macintosh HD (by default) containing protected system files, and a volume named Macintosh HD - Data, which stores Home folders and writeable files. If you have customised the standard top-level folder layout in an earlier version of macOS, then you’ll need to establish how that will convert to the new layout, which at the top level is far more inflexible.

For most users, this should pass almost unnoticed until the day when you open Disk Utility and the two volumes are revealed. Sometimes, if your upgrade goes a bit awry and you decide to start again, or perform a re-install, you can end up with multiple data volumes, such as Macintosh HD - Data - Data. Although Catalina may appear to continue working normally, this can also cause strange problems, and needs to be sorted out.

I have explained in detail how this new volume layout works. You may wish to refer back to that, and its PDFs.


If you’re still interested in upgrading to Catalina, and are confident that you can take those issues in your stride, my final warning is that reverting from Catalina to any previous version of macOS isn’t straightforward. It isn’t impossible by any means, but it’s going to take you many hours of careful work. First, the boot disk has to be put back into the simpler volume layout for earlier macOS, so its contents will be completely erased. Then you’ll have to boot in the older version, run the full installation, and migrate or otherwise reinstall all your apps, Home folder(s), and the rest.

If Catalina gets into a real mess, even clean re-installing it isn’t simple.

Is Catalina worth all that risk and effort? If you’re intending to go on to upgrade to 10.16, then it’s probably a wise move. Hopefully Apple will have addressed the problems with Mail and Time Machine by then, but the structural changes to the boot disk, notarization and security issues aren’t going to go away. Experience with all upgrades in the past is that it’s generally easier going up one major version than trying to leapfrog by two or more. And if you’ve delayed thinking about upgrading to 10.15 until now, you’re probably not going to rush to upgrade to 10.16 early either.

Whatever you decide, I wish you success, and hope that your thorough preparations make any journey swift and smooth.