The order can make a difference in some cases.
A bad external data source reference should not freeze the database. Filemaker starts with the first reference and keeps working down the list until it gets a reference that works. If it gets to the end of the list without finding a file to open, you get a file not found dialog box.
Note that if you have a reference that specfies an IP address that is invalid, FileMaker takes a very long time determining that the address is invalid before attempting the next reference in the list. I have a case where the IP address is to an emergency back up server where the files are kept closed unless the primary server fails. I had to make sure the relative path reference was listed first to avoid delays when the file opens.