Hello HomerJ,
You are somewhat correct in your assumption. You do have enough space for the new project but not for the copy procedure. As the copy procedure requires 2 times the amount of storage comparable to the original project sizing. in your case you would need 4GB of free storage to just have the copy complete as the system reserves the 2GB for the new project and it also reserves a swap file of the same size as the source project. So in your case you have insufficient space available for the copy to complete, It is not a project issue in this case but in fact it is a storage issue and if you could free up another 2GB I think you'll find the copy would complete as the other projects have.

Having said that you have a few options in this case. You could export the project to a qcp and then create a new project from the exported qcp file. this may allow you to get around the free space limitations, depending on how much the qcp takes up, however you could create the qcp and then export the original to your archive volume. this should provide sufficient space to complete all tasks to create the new project.

Next you have the option of creating a new blank project and just dragging the entities desired into the newly created project. This would move all the project entities to the new project, however the limitation of this method is that any linkages established in the original project would be lost and need to be recreated as the drag and drop method doesn't copy the originals into the new projects but instead creates new entities that contain all the original data. Since they are new entities they will have new id numbers and thus all the linkages in the original will be broken as the linkage uses id numbers to create the connection. This means your users will need to go back in and recreate the linkages if they need them.

Hope this helps add clarity to the issue you are experiencing,
Dan