diff --git a/scripts/build.sh b/scripts/build.sh
index 5a5de31..05760af 100755
--- a/scripts/build.sh
+++ b/scripts/build.sh
@@ -32,7 +32,6 @@ mkdir -p "${BUILD}"
 
 UEFIPAYLOAD="${BUILD}/UEFIPAYLOAD.fd"
 COREBOOT="${BUILD}/firmware.rom"
-USB="${BUILD}/usb.img"
 EDK2_ARGS=(
     -D SHELL_TYPE=NONE
     -D SOURCE_DEBUG_ENABLE=FALSE
@@ -91,30 +90,4 @@ then
         "${BUILD}/ec.rom"
 fi
 
-if [ "${MODEL}" != "qemu" ]
-then
-    # Rebuild firmware-update
-    export BASEDIR="system76_${MODEL}_${VERSION}"
-    pushd apps/firmware-update >/dev/null
-      make "build/x86_64-unknown-uefi/boot.img"
-      cp -v "build/x86_64-unknown-uefi/boot.img" "${USB}.partial"
-    popd >/dev/null
-
-    # Copy firmware to USB image
-    mmd -i "${USB}.partial@@1M" "::${BASEDIR}/firmware"
-    mcopy -v -i "${USB}.partial@@1M" "${COREBOOT}" "::${BASEDIR}/firmware/firmware.rom"
-    if [ -e "${BUILD}/ec.rom" ]
-    then
-        mcopy -v -i "${USB}.partial@@1M" "${BUILD}/ec.rom" "::${BASEDIR}/firmware/ec.rom"
-    elif [ -e "${MODEL_DIR}/ec.rom" ]
-    then
-        mcopy -v -i "${USB}.partial@@1M" "${MODEL_DIR}/ec.rom" "::${BASEDIR}/firmware/ec.rom"
-    fi
-    if [ -e "${MODEL_DIR}/uecflash.efi" ]
-    then
-        mcopy -v -i "${USB}.partial@@1M" "${MODEL_DIR}/uecflash.efi" "::${BASEDIR}/firmware/uecflash.efi"
-    fi
-    mv -v "${USB}.partial" "${USB}"
-fi
-
 echo "Built '${VERSION}' for '${MODEL}'"
diff --git a/scripts/usb.sh b/scripts/usb.sh
new file mode 100755
index 0000000..2c0d8b8
--- /dev/null
+++ b/scripts/usb.sh
@@ -0,0 +1,51 @@
+#!/usr/bin/env bash
+
+# Create a USB image file that can be written to a USB flash drive to update
+# firmware on another computer.
+
+if [ -z "$1" ]; then
+    echo "$0 <model>" >&2
+    exit 1
+fi
+MODEL="$1"
+
+if [ "${MODEL}" = "qemu" ]; then
+    echo "can't build USB image for flashing QEMU" >&2
+    exit 1
+fi
+
+if [ ! -d "models/${MODEL}" ]; then
+    echo "model '${MODEL}' not found" >&2
+    exit 1
+fi
+MODEL_DIR="$(realpath "models/${MODEL}")"
+
+DATE="$(git show --format="%cd" --date="format:%Y-%m-%d" --no-patch --no-show-signature)"
+REV="$(git describe --always --dirty --abbrev=7)"
+VERSION="${DATE}_${REV}"
+
+BUILD="$(realpath "build/${MODEL}")"
+if [ ! -e "${BUILD}/firmware.rom" ]; then
+    echo "'${BUILD}/firmware.rom' not found" >&2
+    exit 1
+fi
+
+USB="${BUILD}/usb.img"
+
+# Rebuild firmware-update
+export BASEDIR="system76_${MODEL}_${VERSION}"
+make -C apps/firmware-update "build/x86_64-unknown-uefi/boot.img"
+cp -v "apps/firmware-update/build/x86_64-unknown-uefi/boot.img" "${USB}.partial"
+
+# Copy firmware to USB image
+mmd -i "${USB}.partial@@1M" "::${BASEDIR}/firmware"
+mcopy -v -i "${USB}.partial@@1M" "${COREBOOT}" "::${BASEDIR}/firmware/firmware.rom"
+if [ -e "${BUILD}/ec.rom" ]; then
+    mcopy -v -i "${USB}.partial@@1M" "${BUILD}/ec.rom" "::${BASEDIR}/firmware/ec.rom"
+elif [ -e "${MODEL_DIR}/ec.rom" ]; then
+    mcopy -v -i "${USB}.partial@@1M" "${MODEL_DIR}/ec.rom" "::${BASEDIR}/firmware/ec.rom"
+fi
+if [ -e "${MODEL_DIR}/uecflash.efi" ]; then
+    mcopy -v -i "${USB}.partial@@1M" "${MODEL_DIR}/uecflash.efi" "::${BASEDIR}/firmware/uecflash.efi"
+fi
+mv -v "${USB}.partial" "${USB}"