You edit a block and get the following message:
So, why can I not edit linked to file blocks?
First, let’s define the concept of blocks. There are block instances and block definitions. What you see inserted in the drawing sheet area are block instances (Fig. 1)
All the same block instances belong to a block definition. The block definition is “embedded” into the drawing file and is only shown in the drawing feature tree (Fig. 2)
When you edit a block (by right clicking on an instance) you are editing the block definition (Fig. 3) . A change to the block will propagate to all the block instances in the drawing.
A block instance linked to its file (Fig. 4 ), makes the block file a block definition.
Editing a linked block from a drawing could have severe consequences. Imagine a linked block file that is used in many drawings. This block file could be referenced by hundreds of drawings and in countless instances. You are working on a drawing, need to make a change to the block instance for this particular drawing or circumstance, you edit the block, change it. What is suppose to happen? There could be 2 scenarios:
1) Change the block file. If this happens, then all the drawings in which this block is used will also change. Clearly this is a dangerous proposition
2) Change only that specific block instance. This is a reasonable option. However, this block is linked to a file. Therefore, it cannot be different from the block definition/file.
Hence, in order to edit a linked block, you must unlink the block instance from the block file. Now, the block definition will exist only in the drawing. You can now proceed and edit this block.