I'm trying to upgrade my 21.5" Mid 2010 iMac from Yosemite to High Sierra which is the highest operation system that I can use. However, when I try to run the installer, I get the message that I need to change the Partition Scheme on my startup drive to GUID. Of course, Disk Utility won't let me do this because I'm installing the upgrade onto the startup drive. How do i get around this problem? I can't find the original DVD discs.
If your Mac is currently booting properly or if you have access to another Mac, then create a bootable macOS 10.13 USB installer using these instructions:
https://support.apple.com/en-us/HT201372
Once you boot from USB installer you will need to erase the whole physical drive as GUID partition and MacOS Extended (Journaled). Use this Apple article as a guide to getting the physical drive to appear on the left pane of Disk Utility so the physical drive can be erased:
https://support.apple.com/guide/disk-utility/erase-and-reformat-a-storage-device-dskutl14079/mac
Just make sure to have a good backup of your data before erasing the drive so you don't lose any important files.
If your Mac is currently booting properly or if you have access to another Mac, then create a bootable macOS 10.13 USB installer using these instructions:
https://support.apple.com/en-us/HT201372
Once you boot from USB installer you will need to erase the whole physical drive as GUID partition and MacOS Extended (Journaled). Use this Apple article as a guide to getting the physical drive to appear on the left pane of Disk Utility so the physical drive can be erased:
https://support.apple.com/guide/disk-utility/erase-and-reformat-a-storage-device-dskutl14079/mac
Just make sure to have a good backup of your data before erasing the drive so you don't lose any important files.
You cannot do it without the original DVDs or a Snow Leopard retail upgrade DVD which you can find on Ebay. You will need to do a complete erase of the HD and format it is as Mac OS Extended (Journaled) with a GUID partition. This will erase all data on the HD!!!!!!!!! The SL DVD will look like the following.
To be honest though you are throwing money and effort on an extremely obsolete machine with a value close to $0. I strongly recommend focusing those efforts and money into buying it's replacement.
If you cannot afford a new iMac there are many options available to you.
The only reason I want to upgrade this machine is so I can upgrade my iPad and iPhone to IOS 14. I have a lot of books and music on this computer which will still recognize IOS 12. I will have to hunt around for the startup DVD and maybe I can find it. If I can’t then I will just wait to upgrade my iPad and iPhone. I’m waiting until Apple comes out with one of their new ARM iMacs before replacing this one.
I have a 1986 original Mac and two iMac original computers and they all still work. I don’t upgrade hardware very often.
Help! How do I change my startup disc to GUID Partition Scheme?
