I’m trying to update some computers to Windows 10. I have an iso file with windows 10 on it. I could write it to a dvd, but most of the computers I’m updating no longer have dvd players. I’d like to write it to a usb drive and boot from that. Over the past few days, I’ve found that this is a problem with windows 10 because there is one file, sources/install.wim that’s over 4gb. My systems will only boot from a fat or fat32 partitioned drive. On those drives, 4gb is the maximum file size. I also normally work on a mac, so I was trying to find a way to make these bootable usb drives on my mac. I was not successful. The key to making this work is to use a windows program called split-windowsImage. Here’s what I did, though making the NTFS filesystem on my usb drive was not necessary.

On my mac, I had a 1tb drive sitting around, so I partitioned it with two FAT32 partitions.

$ diskutil partitionDisk disk3 MBR fat32 DOS 300gb FAT32 NTFS 300gb
Started partitioning on disk3
Unmounting disk
Creating the partition map
Waiting for partitions to activate
Formatting disk3s1 as MS-DOS (FAT32) with name DOS
512 bytes per physical sector
/dev/rdisk3s1: 585794432 sectors in 9153038 FAT32 clusters (32768 bytes/cluster)
bps=512 spc=64 res=32 nft=2 mid=0xf8 spt=32 hds=255 hid=2 drv=0x80 bsec=585937504 bspf=71509 rdcl=2 infs=1 bkbs=6
Mounting disk
Formatting disk3s2 as MS-DOS (FAT32) with name NTFS
512 bytes per physical sector
/dev/rdisk3s2: 1367186816 sectors in 21362294 FAT32 clusters (32768 bytes/cluster)
bps=512 spc=64 res=32 nft=2 mid=0xf8 spt=32 hds=255 hid=585937507 drv=0x80 bsec=1367520669 bspf=166893 rdcl=2 infs=1 bkbs=6
Mounting disk
Finished partitioning on disk3
/dev/disk3 (external, physical):
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:     FDisk_partition_scheme                        *1.0 TB     disk3
   1:                 DOS_FAT_32 DOS                     300.0 GB   disk3s1
   2:                 DOS_FAT_32 NTFS                    700.2 GB   disk3s2

I have NTFS for mac on my laptop, so I then formatted the NTFS partition as an NTFS drive. Then I mounted my windows 10 iso file and copied the files to both partitions. (Again, I didn’t need to do this, but I didn’t realize that at the time.)

be:CPBA_X64FRE_EN-US_DV9 $ sudo cp -r . /Volumes/DOS
cp: /Volumes/DOS/./sources/install.wim: File too large
be:CPBA_X64FRE_EN-US_DV9 $ sudo cp -r . /Volumes/NTFS

At this point, I took the external drive to my windows laptop and started powershell as an administrator. The DOS drive came up as D and the NTFS drive came up as E.

I moved to the E:\sources directory and ran this:

PS E:\sources> Split-WindowsImage -imagepath install.wim -splitimagepath "D:\sources\install.swm" -FileSize 1024

It took about 10 minutes and then showed the log file. After that I was able to boot windows 10 and install it.