diff --git a/README.md b/README.md
index 9048901..cda4073 100644
--- a/README.md
+++ b/README.md
@@ -27,14 +27,17 @@ Create an empty hard disk using `qemu-img`, changing the name and size to prefer
 qemu-img create -f qcow2 MyDisk.qcow2 64G
 ```
 
-and add it to the end of `basic.sh`:
-```
-    -drive id=SystemDisk,if=none,file=MyDisk.qcow2 \
-    -device ide-hd,bus=sata.4,drive=SystemDisk \
-```
-> Note: If you're running on a headless system (such as on Cloud providers), you will need `-nographic` and `-vnc :0 -k en-us` for VNC support.
+> Note: If you're running on a headless system (such as on Cloud providers), you will need to add `-nographic` and `-vnc :0 -k en-us` (for VNC support) to the end of `basic.sh`.
 
-Then run `basic.sh` to start the machine and install macOS. Remember to partition in Disk Utility first!
+Then run `basic.sh` to start the machine and install macOS, setting the
+`SYSTEM_DISK` environment variable to the path to your freshly created
+system disk image:
+
+```
+SYSTEM_DISK=MyDisk.qcow2 ./basic.sh
+```
+
+Remember to partition in Disk Utility first!
 
 ## Step 2a (Virtual Machine Manager)
 If instead of QEMU, you'd like to import the setup into Virt-Manager for further configuration, just run `make.sh --add`.
@@ -43,4 +46,4 @@ If instead of QEMU, you'd like to import the setup into Virt-Manager for further
 
 You're done!
 
-To fine-tune the system and improve performance, look in the `docs` folder for more information on [adding memory](docs/guide-performance.md), seting up [bridged networking](docs/guide-networking.md), adding [passthrough hardware (for GPUs)](docs/guide-passthrough.md), and enabling sound features.
\ No newline at end of file
+To fine-tune the system and improve performance, look in the `docs` folder for more information on [adding memory](docs/guide-performance.md), seting up [bridged networking](docs/guide-networking.md), adding [passthrough hardware (for GPUs)](docs/guide-passthrough.md), and enabling sound features.
diff --git a/basic.sh b/basic.sh
index 5902877..12343b7 100755
--- a/basic.sh
+++ b/basic.sh
@@ -6,6 +6,16 @@ OVMF=$VMDIR/firmware
 #export QEMU_AUDIO_DRV=pa
 #QEMU_AUDIO_DRV=pa
 
+[[ -z "$SYSTEM_DISK" ]] && {
+    echo "Please set the SYSTEM_DISK environment variable"
+    exit 1
+}
+
+[[ -r "$SYSTEM_DISK" ]] || {
+    echo "Can't read system disk image: $SYSTEM_DISK"
+    exit 1
+}
+
 qemu-system-x86_64 \
     -enable-kvm \
     -m 2G \
@@ -26,3 +36,5 @@ qemu-system-x86_64 \
     -device ide-hd,bus=sata.2,drive=ESP \
     -drive id=InstallMedia,if=none,file=BaseSystem.img \
     -device ide-hd,bus=sata.3,drive=InstallMedia \
+    -drive id=SystemDisk,if=none,file="${SYSTEM_DISK}" \
+    -device ide-hd,bus=sata.4,drive=SystemDisk