If a referenced drawing cannot be located when you open a drawing, several options available to you. If a nested reference refers to itself or the drawing in which it's references, an error is displayed.
The program stores the folder path of the referenced drawing. Each time you open or plot the drawing, or use the Reload option in the External References palette to update the xref, the program checks the folder path to determine the name and location of the referenced drawing file.
If the name or location of the drawing file has changed, the program cannot locate or reload the xref, and it displays an error message that displays the folder path and name of the missing drawing file.
In the drawing, at each insertion of the missing xref, the program displays text that displays the folder path of the missing xref. You can use the XREF Path option to update or correct the path.
Along with error messages being displayed at the Command prompt, a task dialog box might be displayed that allows you to ignore all missing xrefs or update their folder locations. You can use the External References palette to update the locations of the unresolved references.
To avoid these errors make sure that when you transfer or distribute drawing files that have xrefs attached, you also include all the referenced files.
When a drawing is opened and a nested xref is loaded, the program attempts to find the xref in the original xref path first. If the xref is not found, the following search is initiated in the order shown:
This search order helps ensure that revisions made to the xref are reflected in the current drawing, and also makes it possible for the xref to be found if its folder path has changed.
A drawing that contains a sequence of nested references that refers back to itself is considered a circular reference. For example, if drawing A attaches drawing B, which attaches drawing C, which attaches drawing A, the reference sequence A>B>C>A is a circular reference.
If the program detects a circular reference while attaching an xref, a warning is displayed asking you if you want to continue. If you respond with yes, the program reads in the xref and any nested xrefs to the point where it detects the circularity. If you respond with no, the process is halted and the xref is not attached.
If a circular reference is encountered while loading a drawing, an error message is displayed and the circular reference for the current session is broken. For example, if you have the circular reference A>B>C>A, and you open a.dwg, the program detects and breaks the circularity between c.dwg and a.dwg. The following error message is displayed:
Breaking circular reference from C to current drawing.