The Master Boot Record (MBR) contains only four entries (slots) for four Primary partitions one of which can be an Extended partition. This partition will contain Unallocated space within it where you can create unlimited number of Logical partitions.
Depending on current Basic MBR Hard Disk layout there may be different ways of creating additional partitions.
Note! It is not recommended to convert critical partitions like one with boot files or main OS (in case of multi-boot system). Depending on disk configuration it can cause boot problems.
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