Can I delete iPhoto Library if it was migrated to Photos Library ?
To me it seems that they occupy the same space and this could free space on my Mac.
MacBook Air 13", macOS 10.15
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You can delete the old iPhoto Library (or better, remove it an external volume as a backup). Photos does not need the old iPhoto Library. But you will not gain much free storage, because the photos in your new Photos library are duplicates of the photos in the old iPhoto Library, and they are stored efficiently, so they will use the storage only once. See: Photos saves disk space by sharing images with your iPhoto or Aperture libraries - Apple Support