Hi, i wrote: > > - Compromise is to set the boot flag on a dummy partition of type 0x00. > > This is barely UEFI-compliant because the specs say that a partition of > > type 0x00 shall be regarded as non-existent.
Pascal Hambourg wrote: > - I had to use the old fdisk version from Wheezy because newer versions and > other partition editors would not allow to set the boot flag on an empty > partition entry. You never know what kind of user-friendly abstraction a partition editor will apply. So portable and future-safe is only the manipulation on byte level. The layout of the MBR partition table can be learned from https://en.wikipedia.org/wiki/Master_boot_record#Sector_layout "Structure of a classical generic MBR" https://en.wikipedia.org/wiki/Master_boot_record#Partition_table_entries Partition entries are 16 bytes of size and number 1 begins at offset 446. The boot/active flag sits in the first byte of the partition entry. The value of this byte is either 0 or 128 (= 0x80 in hex). We want 128. Further we need to write values for partition start and size. # Choose target disk or image file disk=/dev/sdX # Make a backup of your MBR by backup_path="$HOME"/mbr_backup_of_"$(basename "$disk")" dd if="$disk" bs=512 count=1 conv=notrunc of="$backup_path" # Restore the backup to $disk would be done like this: # dd if="$backup_path" bs=512 count=1 conv=notrunc of="$disk" # If $disk is the system disk: Have a rescue system ready which can # access the backup file and the system disk. # Choose partition and compute start of its entry partno=2 offset=$(expr 430 + "$partno" '*' 16) # First set the partition entry to all zeros. So we can just hop over # those bytes which shall stay 0. This spares us the plight to handle # 0-bytes by shell commands. dd if=/dev/zero bs=1 count=16 conv=notrunc seek="$offset" of="$disk" # The boot/active flag # (The gesture $'\xHH' is described in man bash section QUOTING.) echo -n $'\x80' | dd bs=1 count=1 conv=notrunc seek="$offset" of="$disk" # Start C/H/S = 0/0/1 offset=$(expr "$offset" + 2) echo -n $'\x01' | dd bs=1 count=1 conv=notrunc seek="$offset" of="$disk" # End C/H/S = 0/0/1 offset=$(expr "$offset" + 4) echo -n $'\x01' | dd bs=1 count=1 conv=notrunc seek="$offset" of="$disk" # Sector count = 1 (start LBA is 0) offset=$(expr "$offset" + 6) echo -n $'\x01' | dd bs=1 count=1 conv=notrunc seek="$offset" of="$disk" The task would also be a good execercise for Programmer's First C Program. Have a nice day :) Thomas
