JRSC wrote:
Thank you for this response. I don't have a cable to connect the two macs, so I see two ways forward.
1) I can copy the iphoto library file to an external HD and then copy it to the new Mac. Question on that - there is not a photos folder on the new mac yet (it's a clean computer - nothing on it yet). do I create one? or should I import a few pictures to photos first so one gets created?
2) I can hook up my time machine EHD to the new computer, load time machine and restore JUST the iphoto library, right? will that time machine backup work since the OS versions are about five years and multiple versions apart?
Thanks!
I would use the external drive than to copy the library over. Just ensure, that the drive has the correct file system MacOS Extended (Journaled), or the iPhoto Library may fail to copy because of file errors. If the drive has a different file system, save any documents from the drive elsewhere and reformat it as described here for Aperture - the same will work for iPhoto and Photos: Format external drives to Mac OS Extended before using with Aperture - Apple Support
Enable the "Ignore Ownership Flag" for this drive. This will prevent permission errors, since you are copying between differnet computers with different user accounts.
To create your new Photos Library copy the iPhoto Library into the Pictures folder on your Mac. You will see the Pictures folder when you open the Home folder.
If it does not yet show in the sidebar enable it. While you are in the Finder go to the main menu bar, open the "Finder" menu, then Preferences. Click the Sidebar tab and enable all items you want to show in the sidebar.

Once your iPhoto Library is in your Pictures folder, drag it onto the Photos icon in the Dock. Photos will create a new Photos library from the iPhoto Library.
To your question 2: The procedure described in Pondini#s tip I linked to should work, but I cannot test it on my Macs. They are all running different systems. Since you still have the original iPhoto Library it is much saver to copy the library instead of trying to restore it from a backup.