If I just want my external hard drive for storing data, is it wrong to be partitioned as primary, NOT as extended? Why?
I think a primary hard drive typically contains the OS or system files. an extended drive is for data. If this doesn't help let me know what you're exactly using it for (other than "storing data" which is what all hard drive do I suppose) and what system you're using if you need help setting it up
There's nothing wrong at all with having the disk partitioned as a single primary partition, or with up to 4 primary partitions. Extended partitions are usually only used to overcome the limitation on having only 4 primary partitions dictated by the structure of the partition table in the Master Boot Record. Extended partitions are containers, and always contain one or more Logical partitions. Simplicity is always better, in my view. For some detailed info on extended partitions, see http://en.wikipedia.org/wiki/Disk_partitioning#Extended_partition.