macOS — Windows 11 USB Creation when install.wim Exceeds 4 GB (wimlib split)

Modern Windows 11 ISOs include an install.wim that exceeds 4 GB — above the FAT32 single-file limit. balenaEtcher and dd fail silently or produce an unbootable drive. The correct approach is to copy the ISO contents manually and split install.wim into .swm chunks using wimlib-imagex.

Key Points

FAT32 maximum single-file size is 4 GB; modern install.wim is typically 5–6 GB.
balenaEtcher on macOS fails silently when the ISO contains files exceeding this limit.
wimlib-imagex split produces .swm (split WIM) files the Windows installer detects automatically.
The USB must be formatted FAT32 MBR (not exFAT, not GPT) for UEFI legacy compatibility.
All ISO contents are copied first; install.wim is excluded and replaced with the split .swm files.

Procedure

1. Install wimlib

brew install wimlib

2. Format the USB as FAT32 MBR

diskutil list                        # identify the USB disk, e.g. /dev/disk4
diskutil eraseDisk FAT32 WINUSB MBRFormat /dev/disk4

3. Mount the ISO

hdiutil attach /path/to/Win11.iso    # mounts as /Volumes/CCCOMA_... or similar

4. Copy ISO contents excluding install.wim

rsync -avh --exclude=sources/install.wim \
    /Volumes/CCCOMA_X64FRE_EN-US_DV9/ \
    /Volumes/WINUSB/

Replace the mount path with the actual volume label.

5. Split install.wim into 3 GB chunks

wimlib-imagex split \
    /Volumes/CCCOMA_X64FRE_EN-US_DV9/sources/install.wim \
    /Volumes/WINUSB/sources/install.swm \
    3000

The 3000 argument is the maximum chunk size in MB. This produces install.swm, install2.swm, install3.swm, etc.

6. Eject and boot

diskutil eject /dev/disk4

Boot the target machine from USB. The Windows installer automatically detects install.swm alongside the original install.wim reference.

Why .swm Works

The Windows Setup loader (setup.exe / WinPE) understands Split WIM format natively. When it encounters install.swm in sources/, it reassembles the parts in memory during installation. No user action is required — the file picker shows a single "Windows 11" entry as normal.

Common Mistakes

Mistake	Result
Using balenaEtcher with a modern ISO	Silent failure or unbootable USB
Formatting as exFAT	Some older UEFI firmware can't boot exFAT
Forgetting `--exclude=sources/install.wim` in rsync	Rsync hangs or fails on the oversized file
Splitting to the wrong destination path	Installer can't find `.swm` files

wiki/concepts/apple-silicon-usb-passthrough — when the USB becomes hardware-locked read-only after a dd write, use UTM for recovery
wiki/concepts/kingston-usb-readonly-lock — Kingston DataTraveler read-only controller lock triggered by macOS dd

Sources

daily/2026-05-08.md — Windows 11 USB creation on macOS Apple Silicon; install.wim was 5.4 GB; balenaEtcher failed silently; wimlib split resolved the issue

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