No free MBR slot” may emerge in the following two scenarios:
Scenario one: There are four primary partitions on the disk as the screenshot shows below.
To resolve this problem, we need to convert one of the primary partitions adjacent to the unallocated to logical partition.
It is simple to convert primary partition to logical partition with Partition Wizard.
Select the primary partition to be converted to logical partition and then employ the “Set Partition as Logical” to finish conversion.
Scenario two: There have already been three primary partitions plus one or more neighboring logical partitions. Besides, the unallocated space for creating partition is not adjacent to logical partition as the screenshot shows below.
To resolve this problem, we need to convert the primary partition between unallocated space and logical partition to logical partition. In the case showed above, we should convert I: to logical partition equally by employing “Set partition as logical” function.
After conversion, we can successfully create partition with the unallocated space.