Big Sur goes so far then stops downloading
cannot finish downloading Big Sur 11.1 on my wife's iMac
iMac Pro
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When this happens it is usually because the file got corrupted in the download or because your internet connection was not stable. Begin the process again. Put any downloaded file into the trash and empty it. Make sure your Mac is directly connected to your modem/router via a wired Ethernet connection. Do not use Fi-Fi. stay off the internet during the download. If you are connecting to a download link, use Safari, not Firefox. The download should take around an hour, more or less. Then the installation can take another 45 minutes, more or less.
Be aware that when a new OS is installed you will often have to download a new compliant driver for your printer or it may not work.