BaseSystem.dmg
I have been getting thi message while trying to update from High Sierra to Mojave or Catalina. How can I solve this?
iMac 21.5″, macOS 10.13
Question marked as
What are you actually doing when this error occurs? How are you attempting to upgrade macOS and running the installer?
If you are running the macOS installer from within macOS, then try booting into Safe Mode and run the installer again in case you have some third party software installed which is interfering. If you still get this error, then maybe the installer is corrupt. Delete the installer app and empty the Trash. Then download the macOS installer again making sure to only use the links contained within this Apple article:
How to create a bootable installer for macOS - Apple Support
If you are using a macOS USB installer, then maybe the USB stick is bad and has damaged this installation image file.
Try running the Apple Diagnostics to see if any hardware issues are detected.
Make sure to have a good backup before attempting to upgrade macOS just in case something else goes wrong.
You should always have frequent and regular backups of your computer and all external media (including the cloud) which contains important & unique data.