The Library module only shows folders that you have imported images from. It is not a File Manager.
If you use the Library module to MOVE images from the main drive to the NAS then that folder on the NAS will show up but no other folders on the NAS will be visible. LRs Library module is Not a File Manager.
No real need to Map that NAS as a drive letter. It should still be visible to a File Manager as a Networked devices/drive. All NAS boxes have their own built in operating system that makes them visible to all computers connected to the same LAN.
If you could see the NAS as a drive in the Library module you would have duplicates of every image. I don't think this is something you want to do.
Once you work on a set of images that are located in one folder on your main drive use LR to Move that complete folder to the NAS then those same images will be visible to LR as being on the NAS drive.
English is not my first language, so please bare with me
I understand, but Lightroom can't acces the NAS at all. I tried to copy some photos manually via windows explorer to the NAS and re-import them, but LR doesn't even recognize the NAS or any folder in it. When I want to add the folder inside the NAS to Lightroom manually, LR does see that there is a folder with the letter X but can't access it.
Regarding the workflow: I want to move every project, photo, etc to the NAS (It runs as Raid1) and then delete everything on my local PC. I need the space and don't really trust my single HDD to save my projects for all eternity. This is why I bought the NAS.
So, what I want to do:
1. Add the folder inside the NAS to Lightroom (LR can't do that)
2. Move my entire photography folder to the NAS (so I can still access it via LR. If I'd move the folder via windows explorer to the NAS, I wouldn't be able to relocate the folder via Lightroom, because Lightroom can't access the NAS)
3. Use the NAS as a backup (2x3TB @ Raid1)
4. When I finish a future project on my local SSD I want to move that project with all the files to the NAS via Lightroom
I hope my english is understandable
In the import dialog window that NAS drive should show up whether you have Mapped a drive letter to it or as a Networked device/drive. If it shows in File Explorer it should show in the Import dialog window. Maybe it has something to do with Windows 10. There are many reports of Win 10 not being very compatible with LR.
This could be in fact the problem. The folder on the NAS shows in the windows file explorer but not in Lightroom - well it is in fact shown, but not accessible :/
I posted the same thread in the WD MyCloud Mirror forum: Adobe Lightroom 6 - Path to WD MyCloud Mirror not ... - WD Community
I can't narrow down who is the badboy here: LR or the NAS or Win10
I swapped the WD MyCloud Mirror with a Synology DS215+. Everything works perfect now and as intended.
Conclusion: There is a reason why Synology is the top company in the NAS field.