I have my Photos for Mac library stored on the Drobo 5N. It opens fine (slow), but it works.
When I try to enable iCloud Photo Library, I get this error message:
iCloud Photo Library is only available on Mac OS Standard and Mac OS Extended formatted volumes.
To check the format of a volume, select the volume in the Finder and choose Get Info from the File menu. The type of volume appears in the Format field.
Since you can’t change the block-level file system on the 5N (versus when using iSCSI protocol), I assume this will forever be a limitation and we won’t be able to use libraries stored on the 5N as an iCloud Photo Library synced storage location?