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MacOS installer done thank you could mean a scenario whereby the macOS installer looks like it is done, but the Mac doesn’t proceed to the next stage or continues displaying the installer's screen. If the installation looks successful, you should let the computer restart and then confirm that it starts on the new macOS. In case the computer goes back to the installer or even an error message, you can restart the computer using macOS recovery mode and try to resolve the issue. You can also consider checking the startup disk using Disk Utility if the problem was caused by issues on the disk. It’s advisable to back up your files before you erase the Mac.
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