I want to provide an FYI for anyone using a Drobo and finds their library inaccessible with 'File not found'.
The Drobo FS allows 2 different ways to mount the drive. This actually caused a problem accessing the content on the Drobo, in which the library was inaccessible, and I had to rebuild it unnecessarily.
1.) Use the Drobo Dashboard and 'Mount' the drive. This has always left the drive mounted and accessible at all times.
2.) Open the Drobo in the 'Shared' section under Finder and access the share. I don't know if the disk unmounts after inactivity as I do not use this feature on the mini.
I have found that 'Mounting' the share via the Drobo Dashboard creates a different directory and path than accessing it via 'Shared'. This caused an issue because when I originally setup my library, the 'Mounted' file directory was used. I was unaware of the benefit of leaving that open, so turned that setting off. I restarted the Mac mini, and when I went to play content within Plex, it couldn't find my file. I remounted it via the 'Shared' option and it was still unavailable within the Library, but available within Finder. I was confused so I cleared my library and rebuilt it using the new 'Shared Directory', but it wasn't needed and actually caused more problems later on. If I just remounted the drive through the Drobo Dashboard using the 'Mount' option, it would have worked just fine.
The solution for this is to leave the Drobo Mounted via the Dashboard. Additionally, I have the Drobo Dashboard setup as a login item, so every time the computer restarts, the Drobo Dashboard reopens and always mounts the drive. I'm not sure if this is necessary, but there was no harm in doing this, so why not.
I shouldn't run into this again, but just an FYI for anyone using a Drobo and finds their library inaccessible with 'File not found'. However, because I remade the library using the 'Shared' directory instead of the 'Mounted' directory, I had to yet again recreate my library because the directory changed yet again. Not a big deal since I figured it out, but can be frustrated for someone who isn't sure whats happening.