macOS catalina install error - this disk doesn't use GUID partition scheme (Au Contraire!) I am trying to upgrade an Late 2013 iMac from OS X 10.9.5 to macOS Catalina.
When I run the catalina installer, it hangs a bit and eventually let's me try to click the drive to install on. After clicking furiously around on it, it eventually pops up the error about I need to use the GUID partition map.

Checking diskutil, I'm greeted by this:
$ diskutil list
/dev/disk0
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:      GUID_partition_scheme                        *1.0 TB     disk0
   1:                        EFI EFI                     209.7 MB   disk0s1
   2:                  Apple_HFS Macintosh HD            999.3 GB   disk0s2
   3:                 Apple_Boot Recovery HD             650.0 MB   disk0s3

Ok, confused. Let me just check disk utility:

Also says GUID partition map. But what is that weird unlabeled partition above &quot;Macintosh HD&quot;. Reboot into recovery mode and run disk utility. I see it's &quot;OSX Base System&quot; or something along those lines. Now when I go look at the partition map on THAT disk? Partition? It says &quot;Apple Partiton map&quot; or something like that. I can't change it in the recovery mode. So what's up? Why does disk utility when running the system not show the &quot;real&quot; partition map type? We can clearly see Macintosh HD here shows GUID partition map. Very confused.
I'd like to upgrade this without reformatting/erasing the drive. Hoping there is some doc or oddity I have not discovered to fix this.
Thank you,
Catalina will only install to APFS, which probably means you will have to install High Sierra first - which will install on HFS+ &amp; convert as it runs.
See How can I download an older version of OS X/macOS? for deeper info on this in general [which unfortunately has got rather long &amp; confusing over the years.]
Jumping this far… definitely have a known-good backup before you start.