I can't select my iMac HD because the OS install believes I am using the same drive for Time Machine backups. I am not - I am using a separate Airport capsule.
I am running OS High Sierra.
I saw another user resolved the problem by finding and deleting backups.backups files on their HD but I can't find such a remnant file.
iMac 27″, macOS 10.13
No, sorry that doesn't change my predicament. As mentioned, my Time Capsule is my time machine drive. The installer incorrectly considers my internal HD as my Time Machine drive. I did power off and disconnect it anyway. But you can see the same error persists in the 2 screenshots attached.
Now resolved - I found the file that did not appear in a Spotlight search of ancient backups on the internal HD and deleted it.
Before installing Catalina, power off the Mac then disconnect the Time Machine Drive
"This disk is used for time machine backups" error when installing catalina
