How to transfer iPhoto to external drive

Which system version is running on your Mac? Are you really still using iPhoto or did you migrate to Photos? Independent of Photos or iPhoto, not only the file system format matters, as Niel has pointed out, you must not move the library onto a volume, that has been used for Time Machine backups or is shared over the network. The volume should have a wired connection. Prepare the external volume as described here for Photos: Move your Photos library to save space on your Mac - Apple Support

Drag the library to a volume formatted as Mac OS Extended (Journaled), launch iPhoto with the Option key held down, and choose it.

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