When saving or opening a map document, the following error may be returned:
Unable to save. Check to make sure you have write access to the specified file and that there is enough space on the storage device to hold your document.
Other than the possible causes listed in the error message, some additional causes for this error message are:
In the first scenario, version privileges, user B in this scenario should close the map document, and check for others editing the same MXD, or try reading/writing the MXD once again.
Note: Alternatively, the file can be saved by using 'Save As' and either renaming the file and saving it to the same directory or saving a file of the same name in another directory.
For the second scenario, image references, there are two possible solutions.