Acrobat Pro won't save to external drive

All of a sudden, Acrobat will not save documents to an exernal hard drive. I'm forced to save them onto the local hard driver and transfer or copy them to the hard drive. 

Error message: 

The file may be read-only, or another user may have it open. Please save the document with a different name or in a different folder.

Neither of these apply. Documents are not read only and are saved with a different name.  

All connections are fine and the hard drive works fine with all other apps. 

This function used to work fine.