Migrating Photos AND iPhoto Libraries to external hard drive

How can the combined Photos and iPhoto be larger than the capacity of the Mac?

It can't and is not - the shared photos are simply double counted

And in your case they are on different drive - so in any case you do not add the sizes since they are not on the same drive -- each library does use that much space on the drive it is on

I understand that Photos points to the old iPhoto library

No that is not correct - both libraries use hard links to share access to the same original photos and previews - neither library uses the other library in any way

Photos saves disk space by sharing images with your iPhoto or Aperture libraries - Apple Support

Pay close attention to this paragraph

When Finder reports the file size of your Photos library, it includes all your originals and previews. It may look like your remaining iPhoto or Aperture library is taking up twice the space on your hard drive, but it isn't—your images exist only in one location, even though you may have more than one photo library.

Since you moved one library the source photos for that library were copied to the external drive and exist there since the target for the hard link must be on the same volume as the link - they also exist on the internal drive since they are being used by the iPhoto library - to release the space used by the originals and previews on teh internal drive you will have to move or delete the iPhoto library too - as long as a single hard link addresses a file the space for that file is not released

And yes moving or deleting wither library has no effect on the other library - again the libraries are not linked to each other - they are both linked to the photos -

And when you move the iPhoto library to the EHD it will use the reported space since you have broken the hard links - each library will have its own copy of the originals and previews and on the EHD the space reported will be used by each

LN

When an iPhoto library is initially migrated to a Photos library they both share the same master files on the hard drive even though each library shows it's occupying essentially the same amount of space on the hard drive.

However, when you move either of this two library or both to another volume they stop sharing the master/orginal files and each take up the space they need on the new volume or drive.

Both libraries will work just fine after moving to a new location. However, at no time does a change made to one library affect the other library. They are totally independent libraries is all respects except for sharing the master/original files at the time of migration.

Yes if you are sure you do not need it - how long has it been since you used it?

LN

I'd probably just delete teh iPhoto library then - you could archive it adn hen delete but probably no reason to do that

LN