What started as a quick VirtualBox install turned into a multi‑headed boss fight: USB errors, a sulking kernel driver, a turf war with KVM, and even a Nix syntax bite. Here’s the complete journey and the fixes that finally made my VM boot.

Head #1 — USB devices wouldn’t enumerate

Can't enumerate USB devices
Could not load the Host USB Proxy service: VERR_NOT_FOUND

Root cause: Host USB support + Extension Pack missing, and the user not in vboxusers.

Fix (configuration.nix):

virtualisation.virtualbox.host.enable = true;
virtualisation.virtualbox.host.enableExtensionPack = true;
users.users.<your-user>.extraGroups = [ "vboxusers" ];

Apply:

sudo nixos-rebuild switch
reboot

Verify:

groups | tr ' ' '\n' | grep -x vboxusers
VBoxManage list usbhost

Head #2 — “VirtualBox kernel driver is not accessible”

VERR_VM_DRIVER_NOT_ACCESSIBLE

Root cause (in my case): launching a per‑user VirtualBox build while the system had different environment/udev context.

Checks:

# modules & devices are fine?
lsmod | grep -E 'vboxdrv|vboxnetflt|vboxnetadp|vboxpci'
ls -l /dev/vboxdrv /dev/vboxnetctl # expect root:vboxusers, mode 660

# you should be in the group
id -nG | tr ' ' '\n' | grep -x vboxusers || echo NOT-IN-GROUP

# make sure you're using the system binaries
which VirtualBox
which VBoxManage
VBoxManage -v

Fixes that worked:

  • Use only the system package (avoid a per‑user copy): environment.systemPackages = [ pkgs.virtualbox ];
  • Restart your session so it picks up vboxusers (or loginctl terminate-user $USER, then log back in).
  • Kill stale VBox processes: pkill -f VBox.
  • Quick test: VBoxManage list hostinfo should work; then VBoxManage startvm "YourVM" --type headless.

Head #3 — KVM vs. VirtualBox (VMX root mode)

VirtualBox can't operate in VMX root mode
(VERR_VMX_IN_VMX_ROOT_MODE)

Root cause: the KVM kernel modules had already claimed hardware virtualization.

Permanent fix (configuration.nix):

boot.blacklistedKernelModules = [
 "kvm"
 "kvm_intel" # or "kvm_amd" on AMD
];

Temporary test without rebuild:

sudo systemctl stop libvirtd 2>/dev/null || true
sudo modprobe -r kvm_intel kvm # or: kvm_amd kvm
lsmod | grep kvm || echo "KVM not loaded"

Head #4 — Nix syntax bite – ooops

error: syntax error, unexpected '{', expecting INHERIT

Fix: don’t open a new attrset mid‑file; place options directly in the top‑level { ... } of configuration.nix.

Final validation

# devices
ls -l /dev/vboxdrv /dev/vboxnetctl

# environment
which VBoxManage && VBoxManage -v

# quick health
VBoxManage list hostinfo

# start headless (clear error output)
VBoxManage startvm "ComandoVM" --type headless

# logs (path shown in showvminfo)
tail -n 80 ~/VirtualBox\ VMs/ComandoVM/Logs/VBox.log

TL;DR: Fix checklist

  • Enable VirtualBox host + Extension Pack; add yourself to vboxusers.
  • Use the system VirtualBox binary; kill stale processes; refresh your login session.
  • Disable KVM when you need VirtualBox (they can’t share VT‑x).
  • Keep configuration.nix tidy — one misplaced brace can block a rebuild.

After one last reboot with KVM out of the way, the VM finally powered on. The hydra, at long last, was tamed.

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