Q: Photos app basically doubled my 75GB iphoto library so it did not use the same master files as iPhoto.

Well if it did then you're the first. As exhaustively explained on this forum and elsewhere, Photos and iPhoto now use Hard Links to point to the same files. Put another way, they both report being 75 gigs in size, but it's the same 75. So why do you think it actually doubled the disk space used?

Well, one of my customers has the same issue. When I look at the pictures folder, it contains 3 files: Photo Booth, iPhoto and Photos library. The size of the 2 big files together is almost exact the same as the total folder size. Looks as if the size of the iPhota and Photos library is added... Is it usefull to rebuidl the iPhoto library, or doing the sam with the Photo's library?

Terence, with all due respect, your assertion "... you're the first..." is simply not true. I myself asked this question about a month ago and ended up being vaguely insulted by you and another high-level responder.

Photos turned my approx. 3,000 pictures into over 10,000. And someone else has posted about this problem too.

It's time for this problem to be acknowledged and addressed.

Only vaguely?

Photos turned my approx. 3,000 pictures into over 10,000. And someone else has posted about this problem too.

One thing is sure: that something has gone wrong here. Are you dealing with masses of duplicates? If so, how exactly did you migrate from iPhoto to Photos?

And frankly, two posts out of millions of users does not suggest a core issue but more likely local glitches.

The size of the 2 big files together is almost exact the same as the total folder size. Looks as if the size of the iPhota and Photos library is added... Is it usefull to rebuidl the iPhoto library, or doing the sam with the Photo's library?

What's reporting the file size?

You can test your libraries to see if they are using the same file on the hard drive or not:

1 - open both library packages and locate the same master in each.  Make sure you have the same master. The easiest way to do that is to download and use Find Any File to search for a specific file name including the extension. In the search results window you can find the master file in each of the two libraries:

2 - open the Terminal application (located in the Applications/Utilities folder) and enter

ls -li

Note there is a space after -li.

3 - drag the iPhoto master image file into the Terminal window and then press the Enter or Return key.

4 - note the number at the beginning of the next line.

5 - enter the command again and drag the Photos master file into the Terminal window and press one of those keys.

6 - note the first number on the next line in the window.

Toad-Hall:~ toad$ ls -il /Users/toad/Pictures/V9.6\ Yosemite\ Library.iphotolibrary/Masters/2014/12/21/20141221-104508/pixeltestphoto_2.jpg

61403483 -rw-r--r--@ 1 toad  staff  7031573 Jun  1 11:21 /Users/toad/Pictures/V9.6 Yosemite Library.iphotolibrary/Masters/2014/12/21/20141221-104508/pixeltestphoto_2.jpg

Toad-Hall:~ toad$ ls -il /Users/toad/Pictures/PHOTOS.photoslibrary/Masters/2014/12/21/20141221-104508/pi xeltestphoto_2.jpg

61403483 -rw-r--r--@ 1 toad  staff  7031481 Feb 25 11:30 /Users/toad/Pictures/PHOTOS.photoslibrary/Masters/2014/12/21/20141221-104508/pi xeltestphoto_2.jpg

Toad-Hall:~ toad$

If the two numbers are the same each library is using the same data on the hard drive for their master.  If it's different then there are two different files on the hard drive.

Thanks to léonie for this test.

hmmm I might be wrong because according to Disk Analyzer my Macintosh HD is 279 GB big altogether when in reality it's capacity is only 240 GB.

That is how the hard links work.  When you create a hard link to a file the linked file will look exactly like the original file. It will show the same file size as the original. Both files will share the same entry in the file table - the inode. They will use the same blocks on the disk, but when the size of folders is computed both files will be included into the calculation with the full size.

That is why both the iPhoto Library and the Photos Library appear to have the same size. The hard linked original image files are full images files for all purposes.

Try the test that Old Toad suggested to check if the originals are hard linked and sharing the storage on the disk.

When you delete either of the original or the hard linked copy, the storage will not be released.  MacOS will only release the storage, when booth files have been deleted.