The problem is that Photos and similar programs (iMovie, Garageband, etc.) are configured for one user with an integrated database. So if you try to allow multiple users access, you may run into corruption issues. By storing your photo library on a NAS server, you effectively allow multiple access to a single user database. Compounding the problem is that NAS hard drives are formatted differently than Apple hard drives, which leads to further problems and increases the possibility of file corruption. Note that this is not a problem for iTunes as it manages its database differently and the content is not bound to a library. Because of this, WD MyCloud and other NAS devices can connect their products to iTunes servers, avoiding corruption issues.
The solution is to create a sparse image, which is a way to partition and format the hard drive and place the photo library on it. This allows the Mac to see it as a local drive rather than a NAS, and provides reasonable access times (at least for my library of 75,000 images and 420 videos). It uses the same method that NAS drives handle Time Machine backups.
So what do you have to do?
Go to Applications, find the Utilities folder and open Disk Utility from there.
In Disk Utility, go to File on the Apple menu bar and choose Blank Image
Then where it says "Save As" make sure the little arrow is pointing up so you can see the directory the content is in.
Now choose where you want to save the image file on the NAS (I saved it in a public folder), give it a name (I called it "Photos") and specify the size of the image file (I specified mine). 500 GB) and make sure it is formatted as Mac OS Extended Journal. Press Save and you should now have a directory set up called Photos.sparseimage.
(Note that this is the first time I've used Disk Utility to save directly to WDMyCloud. Due to limited file storage on my Macmini (which is why I wanted to move my photo library in the first place), I created a small sparse file and drag the image of the file, drag is on the drive and wipe.For some reason the next time I open Disk Utility I can use the capacity of the NAS server to create a sparse image directly to WDMyCloud so I can create the size I want. )
Now when you double click on the scant image you created, a directory called "Photos" (or whatever you want to call it) will be created on your desktop.
Once the directory is set up, you can copy the photo library there. Go to your home directory in Finder and find a folder called "Pictures" where you will find a file called "Photos Library.photoslibrary". Just select it and drag it into the newly created photo gallery. Depending on the size of your library, copying will take some time - it took me about 30 minutes to copy a 100GB file, but of course the time depends on your settings (I'm connecting directly to my NAS via Ethernet).
Once the copying process is complete, open the Photos app by holding the Alt/Option key. When it appears, select the library in the photos directory. If not, select "Other Libraries" and go that way. The next time you open Photos, your last opened library will be saved.
Note the following: Always open the directory before opening the Photos app, otherwise the library will not appear.
As an added enhancement, if you go to photo settings and select the new library as your system photo library, you can also use it with cloud services (if you use them).
If you often work with photos and therefore always want to have access to this catalog, you can open it automatically when you log in. Just go to System Preferences/Users & Groups, then select your profile and click Login Items. Then drag the folder containing your thinned image and then the thinned image and close the window. The next time you log in, the directory opens automatically without you having to make a selection.
NOTE: I've only been using this solution for 48 hours and haven't had any issues since. I found this solution from LaPastenague who posted it elsewhere and another user has had this solution for 3 years with no reported problems.
NOTE: This solution simply uploads your photos so that you can use the storage on your NAS drive to use the app. Due to the way it is set up, only one user can use the library at a time. Anyone who tries to access it while someone else is using it will get a message that the directory is locked. Because of this, the library is not damaged. If other people want to use the library, you must remove the directory from your computer to give them access. I'm putting my drive in a public folder so my wife can upload photos independently of me. However, if that's not a problem or you want separate libraries for different users, you can set that up for everyone in the user's private folder.
Due to the settings, other users cannot share photos from your NAS drive. I first tried to solve this by uploading the photo gallery to the shared images folder on the NAS drive, provided the user can use the NAS software to view the gallery. However, the WD software is not that smart and the results are terrible because it creates images in a completely different way than "Photos" organizes the library, so I have to edit face images, deleted images, thumbnails and many other images and then the photos I want to see are not organized. So I decided I needed to export the images I wanted to share to a folder. For example, I created a folder called "Family Holidays" under the shared images, and within that subfolder a subfolder is created for each holiday I export. The photo in question. While this isn't ideal, since WDCloud sorts photos by export time rather than recording time, you should consider how you want your photos to appear when viewed from WD software on other devices and Make a naming convention that allows for this. It's a lot of work, but at least you can let other people view the photos without compromising the main library or damaging the original image. You can always download any image if you want to save or edit it locally.
Side note: You should now be able to delete your photo library from your computer, freeing up precious hard drive space. However, please note that if you use Time Machine, your photos will no longer be backed up on your Mac after this step. Therefore, you need another hard drive to back up your NAS server. This is the case even if, like me, your NAS has multiple drives in a raid configuration. While this will protect you from disk failure, it will not protect you from file corruption because it is mirrored. I have a WD MyCloud DL2100 with x2 4TB drives. I think I'll buy a third drive and swap it out weekly to solve the problem (which also means I have external storage for my precious pictures in case of fire or theft), but I also use time machine to back up data on all devices more regularly.
I hope it helps. I've been researching for a few days to find a solution that works for me. At one point I thought not being able to store digital media files on my NAS server would be a wasted investment, but now I'm going to create separate directories for use with iMovie so I can start digitizing years of home video tapes!
I'm not a techie so apologies if I don't have the naming conventions right, but hopefully others with a NAS won't have to search the web for solutions to use photos, iMovie, and similar apps with their NAS and server together.