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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.

annoying error message

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:

what the heck

Also says GUID partition map. But what is that weird unlabeled partition above "Macintosh HD". Reboot into recovery mode and run disk utility. I see it's "OSX Base System" or something along those lines. Now when I go look at the partition map on THAT disk? Partition? It says "Apple Partiton map" 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 "real" 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,

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    Catalina will only install to APFS, which probably means you will have to install High Sierra first - which will install on HFS+ & convert as it runs. If you can't get straight to HS, you will have to get El Capitan first. See apple.stackexchange.com/questions/309399/… [which unfortunately has got rather long & confusing over the years.] Jumping this far… definitely have a known-good backup before you start.
    – Tetsujin
    Commented Sep 11, 2020 at 7:27
  • @Tetsujin is correct. Going from Mavericks 10.9 straight to Catalina 10.15 is asking for trouble. You are skipping Yosemite 10.10, El Capitan 10.11, Sierra 10.12, High Sierra 10.13, and Mojave 10.14. The major file system change to APFS was made in High Sierra. Make sure you have a good backup. You might just want to clone the drive to an external drive then install Catalina clean and use Migration Assistant to migrate your data from the external drive.
    – James Brickley
    Commented Sep 11, 2020 at 22:52
  • @Tetsujin thanks. I upgraded to high Sierra without incident. Took a while. Left it running all night but all is well on high Sierra. Haven’t run through the Catalina install yet but it didn’t throw errors when selecting the drive. Convert your comment to an answer and I’ll accept it. Thanks!
    – mikew
    Commented Sep 12, 2020 at 13:59
  • :) Glad you got to HS in one jump - that's useful info for the future.
    – Tetsujin
    Commented Sep 12, 2020 at 17:05

2 Answers 2

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Catalina will only install to APFS, which probably means you will have to install High Sierra first - which will install on HFS+ & 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 & confusing over the years.]

Jumping this far… definitely have a known-good backup before you start.

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There is no need to install another macOS version. you just need a Windows USB flash bootable. After inserting Windows installer USB, restart your Mac, hold ⌥ Option and click on Windows USB bootable. After choosing the language ,click on "Repair" instead of "Install Windows," click "Troubleshoot," open CMD, type in order:

diskpart
list disk
sel dis 0 (remember to use correct number instead of 0; the number of your primary hard disk not USB)
clean

Then restart your Mac and after holding ⌥ Option, try to setup your macOS again. Before setup macOS, remember to use disk utility for erasing the hard drive. After that you can install macOS on that.

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