Unsupported format for photos library
I have copied my photos library to an external seagate hard drive. I have not deleted the library from my computer hard drive until I confirm I can open it from the external drive. When I hold down option and open Photos and navigate to the library on the external hard drive, I receive the following message - "The library “Photos Library 2” is stored on a drive that uses an unsupported disk format. To access this library, copy it to the internal drive on your Mac or to a drive that uses a supported format." What can I do? The hard drive as far as I can tell is working with my Mac. I am able to open other files from the drive.
Thanks!
Stephanie
MacBook Air, OS X Yosemite (10.10.5)