Boot Manager allows for both primary and extended partitions on a single physical drive. Only one primary partition may be active at a time. However, the extended partition will always be active, regardless of which primary partition is active. Logical drives in an extended partition may therefore be accessed by multiple operating systems started from primary partitions or from logical drives in the extended partition. This access is dependent on the operating system supporting the format of the extended partition.
Table "Partition Format Accessibility" illustrates some of the dependencies between operating systems and extended partitions.
Note that it is not possible for an operating system loaded from a primary partition to access any other primary partition on that hardfile; only the logical drive on the operating system's own primary partition and those on extended partitions may be accessed.
Where it is desirable to have a common set of application or data files which may be accessed by different operating systems, a workable strategy is to locate only those files which are specific to the operating system on the appropriate primary partition, while locating common application and data files on logical drives in extended partitions. This implies that the primary partitions should be created to be comfortably large enough to accommodate the desired configuration of the operating system concerned, but no larger.
Since the amount of disk space required to configure any operating system to meet particular needs will vary widely with those needs, readers should consult the installation and configuration instructions for the operating system concerned, in order to determine a reasonable partition size.
When setting up a complex set of operating systems and extended partitions, readers should carefully consider the implications of logical drive letters that each operating system will automatically assign to all logical drives visible to the operating system. This especially applies to applications which reference other applications or data located on different logical drives. The logical drive letters may not be consistent between operating systems; for example, a logical partition formatted for HPFS will be completely invisible to DOS 3.3, but visible though not accessible to DOS 4.0 (CSD UR31300). This may result in different logical drive letters being assigned to other logical drives, depending upon which operating system is currently active in the machine, which in turn holds implications for the configuration of applications and the design of batch files.