Partition Magic full (media fire link)

The best way to understand how partitioning works is by viewing partitions graphically. Posted here is a screen-shot taken from Partition Magic. Here you see 6 hard drives. [Actually, you can't see all of Disk #6.] The first 3 disks are SCSI. The last 3 are IDE/ATA. The legend for this graph is posted here.
Notice that Disk #3 contains both NTFS and FAT32 partitions. Notice, too, how multiple logical DOS drives are always enclosed with-in extended partitions [light-blue outline].

If you look closely, you can see that Disk #6 has no primary partitions. This disk is comprised entirely of an extended partition, containing multiple Logical DOS drives.
This is a trick you can use when adding a new hard drive to your system. If you partition a newly added hard drive so it contains no primary partitions (only Logical DOS drives within an Extended partition) ..

.. it will prevent the newly added disk(s) from usurping drive letters from Logical DOS drives already installed in your system. [Because Windows assigns drive letters to all primary partitions before it does to any Logical DOS drives.]

Two Common Problems

1. Not using the entire amount of the disk. For example, after partitioning your 120-GB drive into 3 equal parts [you thought], you find you're only using 80 gigs. To avoid this problem:

When creating the Extended partition, ensure you use the *entire* amount/size of whatever is left after creating the Primary partition.
Continue to create Logical DOS drives until *all* the space is used. [Obviously, if you only make one, large primary partition, neither of these items apply.]

2. After partitioning, you later discover one of the partitions didn't "take" [for whatever reason]. As a result, you end up formatting the wrong drive, and losing valuable data. Obviously this is bad, bad, bad. To avoid this problem:

After partitioning, do not immediately format the new partition(s). Instead, reboot and return to FDISK. From there, select your new disk and view the partition information to verify they "took" [Select item #4: Display partition information]. Ensure the partitions and drive letters look the way you expect them to.

I have had to re-do partitions before, because they didn't 'take' the first time. Simply repeat the process. Format only after you are positive that all partitions you created (thought you created) look the way you expect them to.

Drive Letters

Windows assigns drive letters like so:

1st -> Primary partition of Primary-master
Next -> Primary partition of Primary-slave
Next -> Primary partition of Secondary-master
Next -> Primary partition of Secondary-slave
Next -> Primary partition on SCSI ID 0
Next -> Primary partition on SCSI ID 1 (and so on, until all Primary partitions for all SCSI IDs are assigned)
Next -> All Logical DOS drives of partitions on Primary-master
Next -> All Logical DOS drives of partitions on Primary-slave
Next -> All Logical DOS drives of partitions on Secondary-master
Next -> All Logical DOS drives of partitions on Secondary-slave
Next -> All Logical DOS drives of partitions on SCSI ID 0
Next -> All Logical DOS drives of partitions on SCSI ID 1 (and so on, until all drive letters are assigned to all Logical DOS drives on all SCSI IDs).

The thing to note here is that *all* Primary partitions receive drive letters before ANY Logical DOS drives do.