external drive
Why can't I open photos on an external drive. I have opened other things like documents on that external drive.
iMac 21.5″, macOS 10.15
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Is your Photos Library on the external drive or a folder with photos? Photos cannot work with a library on a volume with an incompatible file system format or where backup software is limiting the access rights. Also the library must not be kept on a network share or in a synced folder like DropBox or Google Drive.
To keep your Photos Library on an external volume, prepare the external drive as described here:
Move your Photos library to save space on your Mac - Apple Support