[pantulis]

Except that storing your iCloud Photo Library on an external drive is a PITA, as multiple daemons (photoanalysysd being one of them) will randomly activate themselves to do their shit, making it difficult to predict when the drive will not be able to be gracefully ejected. It needs to be a permanent external disk.

[r00fus]

We're talking iMac - so it's effectively permanent external storage.

[stephenr]

... in macOS all drives always need to be "gracefully ejected".