iPhoto vs Photos confusion

The original iPhoto Library and the new Photos Library are totally independent of each other. The hard links to the originals and versions in the iPhoto Library are independent files. They have the same size as the original files in iPhoto, and the will continue working, even if you delete the iPhoto library. The storage is saved, because the hard linked file is stored onto of the original file, because the data are identical. The files are sharing the storage. That is why the sum of the storage for your Photos library and the iPhoto Library is much less than sum of the file sizes. Some files are sharing the storage. if you delete one library the other will keep working. The storage for hard linked files will only be released, of both the original and the linked copy are deleted.

See: Six Colors: The (hard) link between Photos and iPhoto

When I upgraded what happened to Originals folder in iPhoto package structure and what happened to Masters folder in Photos...?

The originals in iPhoto are still there, and you have now a new Masters folder in Photos. Both contain original image files. It is hard to tell from looking at a file, if if it is a hard linked file. They are not references like Aliases.

I can't have 2 sets of originals in iPhoto and Photos as my drive isn't big enough.

That is the magic of hard links.

You now have two sets of original image files. They are identical. One set is in your Photos Library and one set in your iPhoto Library. But because they have been created as hard links macOS is storing the two sets of files on the same disk blocks. They are using the storage only once.

That is why you can store two large library on one small drive.

Now I see I have iPhoto Library.migratedphotolibrary which is sized at 187 Gb, and a Photos Library.photoslibrary file sized at 208 Gb. My drive is only 300Gb in total...

You can safely delete your iPhoto Library.migratedphotolibrary. It will not free much storage, because the linked files in Photos Library are using the same disk blocks.

I'd keep a copy of the iPhoto Library.migrated, however on a backup drive, just in case.

Folders are not a place on disk - files are bits on disk - the disk directory tells programs how to find the bits that make up a file - using hard links the same bits can appear in different files without being duplicated -- this really is not the place to get in depth Computer Science training - what is critical (even if you do not understand it) is what has been posted multiple times in this thread

The Photos library and iPhoto library are totally separate and there is no connection between them and either can be deleted with no effect on the other

To learn more read the links provided or google for more information or take a local Computer Science Disk Hardware class

LN

The Photos library and iPhoto library are totally separate and there is no connection between them and either can be deleted with no effect on the other

They both use hard links to the original photos and the [reviews so the space is not duplicated and deleting either one will not save much space - Photos saves disk space by sharing images with your iPhoto or Aperture libraries - Apple Support -- because of the very small space savings it generally is not a good idea to delete either library until it has not been used for several months ( I suggest at least three months) and then to archive it prior to deleting just in case

All new imports go into the library for the program you are using - nothing done in one program affects the other in any way

And as to finder access - that has never been supported or a good idea - with Photos it is not even possible as the files in the library may not be complete as the full files may only be on iCloud - it has always been the best practice to only use iPhoto or Photos or the media browser to access your photos and now is the only way to access your photos - there are no user serviceable parts within either the iPhoto or Photos libraries

And you can move either or both libraries to a correctly formatted external hard drive (it must be a local drive and only be formatted Mac OS extended (journaled) )

LN

But my originals are in my Originals folder in iPhoto package structure are they not?

No - your originals are on disk and the Masters folder has hard links to them - in each library

So if Photos has a reference only (link) it must have a dependency to iPhoto?

It is not a "reference - it is a hard link as I posted and as described in the link I provided - Did you read it?

They both use hard links to the original photos and the previews so the space is not duplicated and deleting either one will not save much space - Photos saves disk space by sharing images with your iPhoto or Aperture libraries - Apple Support --

As clearly and equivalently stated

The Photos library and iPhoto library are totally separate and there is no connection between them and either can be deleted with no effect on the other

It is that simple - both hard link to the same photos - neither has any connection to the other in any way

LN

That is correct

LN