Q: Permissions problems importing iPhoto library into Photos

Make sure ignore ownership of the other drive is checked.

Select the drive where the data is stored and get info on it. At the very bottom left side under permissions window.

Photos can't access a different volume unless this is checked. If the drive is a Time Machine drive, you won't have this option.

I've tried the various steps in the iPhoto library first aid but no luck so far.

Download our Swiss-Army.Knife for library emergencies: Fat Cat Software – iPhoto Library Manager

The free trial option will let you rebuild the iPhoto Library.

Then try to migrate the rebuilt library to Photos.

What is the file system on your second HD with the Masters folder, and how is this HD connected to your MBP?

ok, checked the ownership option but still no dice. léonie, the HD is formatted as Mac OS Extended (Journaled), and is in the bay that used to house the optical drive, so connected via sata..

Exactly like my MacBook Pro. That should work.

Try to rebuild with iPhoto Library Manager.  Here is the help text on rebuilding a problematic library:

       Rebuilding a corrupted iPhoto library

Not sure at what point the problem is occurring. You can view this article, but I don't know if that permission issue is applying to you:

If Photos can’t copy or create a library - Apple Support

If you have iPhoto 9.6.1 in your application folder, you can open it. If you have iPhoto 9.6.1 and you can't open your library, try repairing your library. Hold the command-Option key down and start iPhoto. It will bring you to the first aid tab.

You can also follow Leonie's post.

You're welcome. Thank the Fat Cat Software team for keeping the trial version free for such emergencies