Bootcamp ImageInstall

Once the backup is complete,

• Boot into Internet Recovery using Command+Opt+R - About macOS Recovery on Intel-based Mac computers - Apple Support
• Erase your whole disk in Disk Utility
  • View all devices or only volumes in Disk Utility on Mac - Apple Support
  • Disk Utility User Guide for Mac - Apple Support
• Re-install a clean copy of macOS Catalina - How to reinstall macOS - Apple Support
• Install Windows
  • Install Windows 10 on your Mac with Boot Camp Assistant - Apple Support
  • Boot Camp Assistant User Guide for Mac - Apple Support
• Post any error messages you see
• If both OSes work properly, you can restore files from the USB manually

mek4n wrote:

BC Assistant says i can return my storage back to my drive, so yes to that question.

Let BCA remove the current partition(s). Reset SMC and NVRAM.

I have nothing plugged into my computer, apart from my keyboard and mouse

We should try the standard, uninterrupted installation. The only unknown now is Big Sur 11.3.1. If it does not work, then do you want to consider downgrading to Catalina for the Windows installation and then upgrade to Big Sur?

If you reboot and hold Alt/Option, do you see a OSXRESERVED (or Windows) as an option?

If that is the case, you may just want to save the files you need to the external USB and then erase your whole internal disk and install Catalina and try to install Windows.

Since it is NTFS, there is no real need for Always. The following...

<ImageInstall>
   <OSImage>
      <InstallToAvailablePartition>true</InstallToAvailablePartition>
   </OSImage>
</ImageInstall>

This should be sufficient.

Run BC Assistant, and it should offer to remove both OSXRESERVED and BOOTCAMP. Do you have any external storage connected to the Mac?

Do you have an AutoUnattend.xml file? If yes, can you post the contents of file here or in PasteBin.com?

mek4n wrote:

I can boot into Windows Setup but then i get the ImageInstall error

Does the 'Always' change to 'Never' during the installation? When you see the error message, press Shift+F10, and you should see a command window. Your C: should point to the Bootcamp partition, and X: should be the Installer. Verify the driver letters, go to the Installer partition, check the XML file. If you are using Create+Download and Install, all checked, any changes to OSXRESERVED are lost as BC Assistant will erase/format/rebuild it.

On a 2020 Mac, BC Assistant silently formats the partition to NTFS, which avoids the interaction. Can you check teh file system type on E:

Do you still have the OSXRESERVED partition? What is the year/model of your Mac (do not post the serial number)? What is the current macOS version?

In macOS Finder, if you click on the OSXRESERVED partition, does the structure look similar to following screen shot...

You want to look at disk D:. Can you change to disk D: and check the AutoUnattend.xml?

mek4n wrote:

Is it possible to store my Macintosh HD-Data disk on another mac then transfer it over when I install Catalina

It is not recommended, due to file system differences between various versions of macOS.