8760w Legacy Boot, will not see GRUB, only Win10 bootmgr.exe? Error: 0F3

[drankinatty]

After building PCs for 20+ years I thought I had seen it all, but I cannot get an HP Elitebook 8760w to see the bootloader installed on a Western Digital 1T drive. The latop has all UEFI *completely* disabled in the BIOS (default). It boots windows 10 just fine using bootmgr.exe (there are no UEFI files present on the windows 10 disk) (legacy boot confirmed with both 'bcdedit /enum' and msinfo32 -- which reports 'BIOS Mode Legacy')

The problem is that after an Archlinux install (of which I've done more than 30) the laptop will not see the grub bootloader installed in the MBR. The comical part is I can boot the .iso just fine and then "Choose existing OS" (change 'hd0 0' to 'hd1 0') and then boot grub, and the Arch install just fine. As soon as control passes to grub, I can pull the USB .iso and continue normally.

(I'm writing this post from the laptop which is happily running Arch with a full Plasma/KDE desktop install -- I just can't boot it without booting from the .iso in a USB port - heck of an unintended security feature... The rest of the machine works flawlessly once booted from the iso -- full plasma/KDE5 setup, wpa_supplicant WPA/TKIP wifi, bluetooth, synaptics touchpad, ieee-1394, I even have the console happily running at 1600x900-32bit mode)

I've tried all the suggestions, pulling the battery, holding power on for 20 seconds to clear any phantom power, resetting the BIOS to failsafe defaults -- nothing works, windows continues to happily boot in legacy mode, while attempting to boot the linux install always results on good-ole error (0F3) (no OS found) (Installed BIOS is F.60 Rev.A, I have not installed F.62 Rev.A yet -- the update didn't appear relevant to this problem from its description)

The MBR partitioning is simple /boot, /, home (and a 1G swap):

$ sudo fdisk -l /dev/sda
Disk /dev/sda: 931.5 GiB, 1000204886016 bytes, 1953525168 sectors
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 4096 bytes
I/O size (minimum/optimal): 4096 bytes / 4096 bytes
Disklabel type: dos
Disk identifier: 0xff7d45aa

Device     Boot      Start        End    Sectors   Size Id Type
/dev/sda1             2048 1953525167 1953523120 931.5G  5 Extended
/dev/sda5  *          4096    1028095    1024000   500M 83 Linux
/dev/sda6          1030144  105887743  104857600    50G 83 Linux
/dev/sda7        105889792 1951383551 1845493760   880G 83 Linux
/dev/sda8       1951385600 1953525167    2139568     1G 82 Linux swap / Solaris

grub is installed to /dev/sda with

grub-install --target=i386-pc /dev/sda  # (no errors on install), and
grub-mkconfig -o /boot/grub/grub.cfg

I've read that I might need to boot with UEFI (even though with this model, UEFI was experimental and disabled by default, and --- is disabled for windows 10) This was explained as being due to HP hardcoding the path to the windows bootmgr.efi file (which makes no sense here as that doesn't even exist on the windows install) Nonetheless, I've got another 1T drive I setup as GPT and tried both a 1meg bios_boot partition to boot the GPT install in Legacy mode and I've tried a full UEFI setup (which pressing F9 on this system after enabling UEFI in the bios results in a single blinking underline cell in the center of the screen (nothing more), then a timeout and continuing with the legacy boot order.

For the record, the bios_boot attempt with GPT used the following partitioning:

 # fdisk -l /dev/sda
Disk /dev/sda: 931.5 GiB, 1000204886016 bytes, 1953525168 sectors
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 4096 bytes
I/O size (minimum/optimal): 4096 bytes / 4096 bytes
Disklabel type: gpt
Disk identifier: 00B6A48C-CBDB-4071-A1EC-97FA828A6C26

Device          Start        End    Sectors  Size Type
/dev/sda1        2048       4095       2048    1M BIOS boot
/dev/sda2        4096    1028095    1024000  500M Linux filesystem
/dev/sda3     1028096  105885695  104857600   50G Linux filesystem
/dev/sda4   105885696 1949282303 1843396608  879G Linux filesystem
/dev/sda5  1949282304 1951379455    2097152    1G Linux swap

(Same result - the laptop will not boot from the hard drive, but happily boots the install when booted via the .iso and control is passed to grub by choose "Choose existing OS" from the iso boot menu)

The UEFI setup, used the following:

 # fdisk -l /dev/sda
Disk /dev/sda: 931.5 GiB, 1000204886016 bytes, 1953525168 sectors
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 4096 bytes
I/O size (minimum/optimal): 4096 bytes / 4096 bytes
Disklabel type: gpt
Disk identifier: 00B6A48C-CBDB-4071-A1EC-97FA828A6C26

Device          Start        End    Sectors  Size Type
/dev/sda1        2048       4095       2048    1M BIOS boot
/dev/sda2        4096    1028095    1024000  500M EFI System
/dev/sda3     1028096  105885695  104857600   50G Linux filesystem
/dev/sda4   105885696 1949282303 1843396608  879G Linux filesystem
/dev/sda5  1949282304 1951379455    2097152    1G Linux swap

No luck, just the blinking underline in one character cell in the center of the screen, then the boot sequence skips to attempting a PXE boot, then my good old friend -- error (0F3) reappears.

So here I sit, with a 128G windows 10 drive in the first hard drive bay, a 1T drive with Linux in the other, UEFI completely disabled and DiskLock off. When I want to boot windows, I simply reboot (and roll the time back to local time in the BIOS for windows) and up pops windows 10. To boot linux, I simply shut down, pop the USB stick in the side, power-on (add 6 hours to the system clock) boot from the USB and "Choose existing OS" to boot from the .iso boot menu, which boots grub fine and loads a flawlessly working Archlinux install with Plasma/KDE, or fluxbox or i3 or whatever desktop I feel like using at the time.

So I give up, throw in the towel, swallow my pride, and ask the experts here at HP -- How do you get this laptop to see the grub bootloader and boot in Legacy mode without having to first boot from USB? Recall, windows happily boots in legacy mode, but when I pull the windows drive, install grub to the MBR and attept to boot Linux, the laptop does not see the bootloader on the hard drive, although it will happily boot from the .iso on a USB stick.

So, What say the experts...?

(If I can provide any more info, just ask, I'm happy to do it)