I had this same problem as well. Sadly, iPhoto does not support the user login system used on any network drive (not just the DNS-345) and so cannot properly access the DNS for specific files within the library package.
The only way to deal with this is to put your iPhoto library on an external USB HDD, and then back that up to a second USB HDD.
Having learnt the hard way, my advice is to not put your iPhoto libraries on any network device, or you will lose data.