grub bootloader damage: the first 446 bytes of the device, i.e. the boot program is corrupted, one sector is 512 bytes, first 446 bytes of the boot program
The sync data in memory to the hard disk synchronization data is not written to disk cotton
Specific operation is as follows
Analog damage
dd if = / dev / zero of = / dev / sda bs = 200 count = 1 // bs represents a flow rate control, count represents the number of transmission times, dd command cover
reboot reboot //
The following operation after restarting
Mode into the shell -4.2 #
Perform the following steps
chroot / mnt / sysimage / switch to bash-4.2 #
Further backup using the following conditions: Method a
dd if=/backup/sda.mbr.bak of=/dev/sda bs=446 count=1
exit
reboot
Method two: no backup situation, the reconstruction sector
grub2-install /dev/sda
sync
exit
exit
reboot