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Hidden partition / true hidden partition

If more than one primary C: partitions are present on a hard disk (e.g. DOS 6.22 and Windows 95) the assignment of drive letters to these partitions might lead to problems. These problems can be avoided if all but one of these primary C: partitions are hidden. It is then possible to boot only from the remaining non-hidden partition. The other C: partitions are not visible, i.e. they are considered as unknown partitions. The drive letter C: is thus always automatically assigned to the booted operating system.

In order to hide partitions the value 10h is added to the partition ID. And in order to make a partition visible only this value 10h needs to be subtracted from the partition ID. The OS/2 boot manager uses the same scheme for hiding partitions.

Both in the configuration program Boot-US and in the boot manager Boot-US only partitions which use drive letters can be hidden. Hiding partitions therefore applies only to the following partition IDs: 01h, 04h, 06h, 07h, 0Bh, 0Ch and 0Eh, (see list of recognized partition IDs). All other partitions (e.g. Linux with partition ID 83h) cannot be hid.

True hidden partition:
The simple method of hiding a partition described above does not work with Windows 2000/XP/2003/Vista/2008/7/8/10. This means, even if the partition ID is switched to hidden, the partition is accessible under Windows 2000/XP/2003/Vista/2008/7/8/10. In order to true hide the partition the boot sector is additionally changed. Hence true hiding a partition means to change both the partition ID and the boot sector. Then the partition is not anymore recognized by Windows 2000/XP/2003/Vista/2008/7/8/10.

Comment:
It is possible that Windows 2000/XP/2003/Vista/2008/7/8/10 considers a true hidden partition as unformatted. This can be changed by removing the drive letter from a true hidden partition. This prevents that you accidentally format such an "unformatted" partition thereby deleting all existing data on that partition. Drive letters are assigned or removed within the Windows disk administrator (Computer Management / Storage / Disk Management).