Recently I made some changes on my network. This resulted in chaos so I decided that I reboot my Linux machine. This piece of hardware is already a bit older and in dire need of replacement. There are legacy installations of Windows XP and Windows 98 which are seldom used. Due to the hardware limits this is installed as dual boot.
I restarted the machine and Windows got up even though Linux was the default in the grub bootloader.
In slight panic I started downloading bootable Linux images to rescue my system. Meanwhile I rebooted several times thinking it might be some glitch. And then I saw it: Some message on a USB device in the boot process. I had my stick left plugged in. The grub is installed in the root partition of the master disk whereas the Windows bootlader is installed in the first primary disk. This caused the bios to skip the root partition and jump ahead to the first disk and start from there.