Catalina Read-Only System Volume

If you get the Catalina User Beta or wait until the final release, one thing you will notice. Catalina is much more locked down than previous Mac OS’s. That is kind of a pain, but then again there is not a more secure operating system on the planet. One huge change is Catalina now has a Read-Only System Volume. You cannot change stuff in the Catalina System files, they are read only. I am not sure what that means for Mac Utility Developers, but there is one observation I would like to mention.New Text file

Two Volumes

I installed Catalina on an External SSD hard drive connected via USB 3.1. I named it “Developer”. It all works just fine, until I booted back into my normal Mojave system I noticed two, not one, but two Catalina drives on my iMac Desktop:

Two Hard Drives
Two Hard Drives

At first, I thought I had done something wrong so I erased the drive and reinstalled Catalina. Yes, that is a process, but I had to know how things were really supposed to work. To my surprise, the two drive thing appeared again!

After doing a little thinking I realized since Catalina’s System is protected, the APFS file system sees it as a separate hard drive. It sees your Data as a separate hard drive. Hence the two hard drives showing on the Desktop. I don’t know if this will display similarly when Catalina is on the main internal drive or if it is only because it is installed on an external drive. I will just have to wait and see on that.

However, if you go into System Preferences/Startup Disk in Mojave you only see the System Drive, in this case “Developer”, not “Developer Data”:

System Preferences
System Preferences

This is all a little weird, but maybe Catalina is such that it has to display that way. Or, maybe when it is on the main Mac it hides the Developer Data portion. So, if you have a typical “Macintosh HD”, maybe you will also have a “Macintosh HD Data” or something like that. 

Only time will tell how all this will play out. The important thing to know is Catalina System Volume is protected, read-only. That is a bit of a game changer for Developers. I hope they can work it out to our satisfaction.


I am enjoying exploring the Catalina OS on my external HD. I have installed most of my mission critical Apps and will start doing some testing soon.