Installing Alongside Windows

Companion page to the Windows-prep checklist in Dual-Booting AcreetionOS with Windows — do those five things FIRST (shrink inside Windows, disable Fast Startup, note BitLocker state).

In the installer

Partitions step → Manual.
Locate unallocated space created earlier → Create:
- UEFI machine: root partition (ext4/btrfs) mounted /; reuse Windows' EFI partition mounted /boot/efi without formatting.
- BIOS machine: root partition with boot flag on msdos table.
Optional swap or plan zram instead.
Summary screen: confirm NOTHING says "format" on NTFS partitions.

After first boot

sudo os-prober                       # should print Windows Boot Manager
sudo grub-mkconfig -o /boot/grub/grub.cfg

GRUB menu now lists both systems; default selection + timeout live in /etc/default/grub (GRUB_DEFAULT, GRUB_TIMEOUT), then regenerate config again.

Verification checklist

Both entries boot cleanly (test Windows once now, not during an emergency)
Clocks agree — apply the registry fix from Dual-Booting AcreetionOS with Windows
BitLocker re-suspended→enabled without complaints
efibootmgr shows sane order (Firmware Settings: BIOS & UEFI)

When Windows Update stomps GRUB (it happens)

Boot ISO → Recovery → GRUB reinstall/boot repair — regenerates entries including Windows detection. Two minutes.

Removing AcreetionOS later

Delete its partitions in Windows Disk Management, expand Windows volume into the space, then repair Windows bootloader (bcdboot) or just reorder UEFI entries.