You may face several instances when you can find your Windows hard disk partitions inaccessible with 'Unknown' partition type. Some of them are here below:
1.) Your Windows-based computer system gets corrupted due to virus attack. The next time, when you boot the computer, it doesn't boot and Fdisk displays the partitions with 'Unknown' type
2.) You install a third-party software, which creates an unknown partition to image the operating system. You cannot delete such partition because it is currently in use by the software
3.) You try to install Windows, but the Setup declares the partition with 'Unknown' type
