the /fixboot and /fixmbr command is safe all they do is repair the boot sector and the Master Boot Record to there original configurations.
Descriptions of each
fixboot drive name:
Use this command to write the new Windows boot sector code on the system partition. In the command syntax, drive name is the drive letter where the boot sector will be written. This command fixes damage in the Windows boot sector. This command overrides the default setting, which writes to the system boot partition. The fixboot command is supported only on x86-based computers.
fixmbr device name
Use this command to repair the MBR of the boot partition. In the command syntax, device name is an optional device name that specifies the device that requires a new MBR. Use this command if a virus has damaged the MBR and Windows cannot start.
Warning This command can damage your partition tables if a virus is present or if a hardware problem exists. If you use this command, you may create inaccessible partitions. We recommend that you run antivirus software before you use this command.
You can obtain the device name from the output of the map command. If you do not specify a device name, the MBR of the boot device is repaired, for example:
If the fixmbr command detects an invalid or non-standard partition table signature, fixmbr command prompts you for permission before it rewrites the MBR. The fixmbr command is supported only on x86-based computers.
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