Hello Lemut, and welcome to Seven Forums.
Could you post a screenshot of your Disk Management (diskmgmt.msc) window showing the layout of all of your HDDs first? This way we will be able to give you a more accurate option to use or try.
Yep, you are correcy. The error you are getting indicates that the HDD you are trying to delete/format/etc.. is a "system" drive, and that is why it will not letting you.
The tutorial below can help show you how to make sure that system protection is not turned on for the HDD that you are trying to delete.
System Protection - Turn On or Off
The tutorial below can help show you how to make sure that you do not have a page file, or to remove the page file, for the HDD that you are trying to delete.
Virtual Memory Paging File - Change - Vista Forums
Hope this helps for now,
Shawn