An error occurred while copying the Windows installation files

I have had this problem and done some investigatio. The actual ISO has increased in size and when the boot camp assistant formats the drive as FAT32 which has a file size limitation of 4gb unfortunately the install.wim in the sorces folder is bigger than 4gb hence it fails. The only work around is to download windows 10 1803 then update within window. I have reported this to both apple an Microsoft. Hope this helps

The latest version excedes the file limit for fat32 you have to for the time being install version 1803 then upgrade within window.Be warned 1809 has a lot of bugs so I would stick with 1803 for the time being

After Step 5, you also need to add the Bootcamp drivers package to OSXRESERVED, before you execute step 6. 🙂

A simpler solution is to use W10 1803 instead of W10 1809.

Do not manually create a partition for Bootcamp/Windows. It will cause more problems. BC partitions should only be cerated/removed by BCA, unless they fail to do so.

The AutoUnattend.xml file is invalid.

If you want to install wind 10 1809 rather than 1803 then have a look at this thread

Boot Camp error: An error occurred while copying the Windows installation files

ibrahim.gaafar wrote:

where i can get or download Bootcamp drivers package ??

and how i can add or install it ??

BC Assistant -> Action -> Download. They are not 'installed', but copied to the OSXRESERVED partition.

I also have problems like you and I have not found a solution 😢

Did the BC Assistant build your OSXRESERVED partition which is being used to install Windows?