What is "bootsect.bak" file?

What is Bootsect.bak backup file? Is this file a copy of boot loader or boot sector? if it's a boot sector's copy, why it's needed? because MBR is a fixed sector.

Solution 1:

Bootsect.bak is a backup of the primary Bootsect.dos. It is created by default by old Windows operating systems when any boot modifications occur. It can be safely deleted, as it is only a back-up unlinked to the actual boot sector.

Tip: Bootsect.dos is used to boot into the non-Windows NT operating systems. Bootsect.dos is created for example by Windows NT Setup. The boot sector found in sector 0 of drive C is copied into Bootsect.dos before Setup writes the Windows NT boot sector. The if you want to dual-boot with previous operating systems (DOS, W95, W98), the file is required, because it's their actual boot sector.