HDM 2011 Suite - Not allowing to create Primary Partition

I have a 1.5 TB SATA drive divided into one primary partition and an extended partition with two partitions.

Additionally I have a 20GB PATA drive which is one (currently blank) Primary Partition, and I have a 500 GB USB Portable drive which I was using to copy files off of the previous hard drive (160GB) which had become too small, and which is also seen by Windows XP as one 500 GB Primary Partition.

The 160 GB SATA drive was previosly divided like the 1.5 TB one is not... one primary partition with Windows XP SP3 on it, and an extended partition divided into two segments.... one for Programs and one for Data.

After I re-installed Windows XP on the 1.5 TB drive, I erased the contents of the Primary Partion on the 160 Gb drive (where previously Windows XP was installed), and after I discovered that HDM 2011 would not allow me to convert the extended partition (the rest of the drive) into a Primary Partition I deleted the Primary Partition, copied all the data from the Extended partition onto the portable drive and deleted the extended partition.

Now I have a unallocated 160GB hard drive with no partitions and what I want to do is make it one Primary Partition of 160 GB on which to store backups.

Neither HDM 2011 Suite OR Partition Manager 9.0 Professional will allow me to create a Primary Partition on this drive... only an Extended Partition.

So I have two questions.

1. Why can't I create a Primary Partition, and how can I create one?
2. Is there any reason I should *want* to have a 160 GB Extended Partition
without any Primary Partition on this drive?

Any thoughts, insights, suggestions, comments would be welcomed!