Partitioning errors and GRUB misconfigurations often prevent successful multiboot setups with OpenBSD and Linux. These issues can block system access or result in OpenBSD failing to boot. Addressing partition alignment, bootloader settings, and disk layout ensures reliable multibooting and reduces the risk of data loss.

Method 1: Configure Partitions and Bootloaders for OpenBSD and Linux

Step 1: Prepare the disk layout carefully before installing any operating system. OpenBSD and Linux have different requirements and partitioning schemes. OpenBSD prefers to be installed on a primary MBR partition and manages its own sub-partitions within a disklabel. Linux can use either MBR or GPT, but GRUB’s behavior may vary depending on the scheme. Use a partitioning tool like gparted or fdisk to create at least one primary partition for OpenBSD and allocate other partitions for Linux. Avoid overlapping partitions and ensure each OS has its own dedicated space.

Step 2: Install OpenBSD first if possible. During installation, select the pre-allocated primary partition for OpenBSD. Allow OpenBSD’s installer to create its internal disklabel partitions (e.g., a for root, b for swap, etc.) within the primary partition. Do not let OpenBSD modify or overwrite Linux partitions.

Step 3: Install Linux on the remaining space. During the Linux installation, choose manual partitioning to avoid overwriting the OpenBSD partition. Assign mount points and format the desired Linux partitions only. When prompted for the bootloader, install GRUB to the disk’s MBR (or EFI partition for UEFI systems), not to a partition boot sector. This allows GRUB to manage booting for both operating systems.

Step 4: Update GRUB’s configuration so it can detect OpenBSD. After Linux installation, boot into Linux and run:

sudo update-grub

This command scans for other installed operating systems, including OpenBSD, and adds them to the GRUB boot menu. If OpenBSD does not appear, manually edit /etc/grub.d/40_custom to add an entry like:


menuentry "OpenBSD" {
    insmod part_msdos
    insmod ufs2
    set root='(hd0,msdos1)'
    chainloader +1
}

Replace (hd0,msdos1) with the correct partition identifier for your OpenBSD install. Then run sudo update-grub again.

Step 5: Reboot and test the multiboot setup. From the GRUB menu, select OpenBSD and verify it boots correctly. If you encounter errors such as "partition not found" or "filesystem unknown," double-check partition types and GRUB configuration.

Method 2: Use OpenBSD's Bootloader with Chainloading

Step 1: Install OpenBSD’s bootloader to its own partition rather than the MBR. During OpenBSD installation, choose the option to install the bootloader to the OpenBSD partition only. This prevents it from overwriting the main bootloader.

Step 2: After installing Linux and GRUB, configure GRUB to chainload the OpenBSD bootloader. Edit the /etc/grub.d/40_custom file in Linux and add:


menuentry "OpenBSD Chainload" {
    set root='(hd0,msdos1)'
    chainloader +1
}

Update GRUB with sudo update-grub and reboot. This method hands control to OpenBSD’s own bootloader, which can be more reliable if GRUB has trouble directly booting OpenBSD kernels.

Method 3: Troubleshoot Common Partition and Boot Issues

Step 1: If installation hangs or partitions are not detected, verify that the disk is using MBR (not GPT), as OpenBSD’s installer has limited support for GPT in some versions. Use fdisk -l in Linux or disklabel in OpenBSD to inspect partition tables.

Step 2: For partition alignment problems, delete and recreate partitions using a tool that writes sector-aligned partitions (such as gparted). Misaligned partitions may cause boot failures or filesystem errors.

Step 3: If GRUB does not see OpenBSD, ensure the OpenBSD partition type is set to A6 (OpenBSD) in the partition table. Use a tool like cfdisk or fdisk to check and adjust this type if needed.

Step 4: For UEFI systems, ensure both Linux and OpenBSD support UEFI booting. OpenBSD’s UEFI support is limited on some hardware, so consider switching to legacy BIOS mode if persistent issues occur.

Careful partition planning and correct bootloader configuration streamline dual-booting OpenBSD and Linux. Keeping backup copies of partition tables and bootloader configs helps recover quickly from mistakes.