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After migrating an iPhoto Library to a Photos App Library, they see actually share space by using hardlinks. So even the Finder says each folder use some amount of space, they are actually sharing it. That only applies if they are in the same volume. For example, both in the internal disk or both in the same partition of the same external drive.

However, migrating could share the actual pictures but each program has specific extra structure. It could be from thumbnails, index and other cached information.

How much extra space do I need, for example, for migrating and 150Gb iPhoto Library in an external drive?

It's important to know if the iPhoto library is in volume, let's say, with 170Gb. Will be the remaining 20Gb enough for the Photo App index?

The situation is particularly important for many Macbook air users as many of them, having small SSD disk, store their pictures in an external drive.

  • When you write Photo, do you mean the Photos app? – jaume Nov 13 '15 at 8:44
  • yes. I'll edit it. – hectorpal Nov 14 '15 at 0:04
  • The space requirements will depend on how many photos you plan to migrate and their resolution. What is the size of the current iPhotos library? – Tony Adams Nov 18 '15 at 16:28
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No overhead at all.

The Photos library created by migrating uses "hard links" that point to the actual files. This causes it effectively just point to the exact same files as the iPhoto library contained.

In my case, my iPhoto library was 40 GB. So is my Photos library. But I only had 10 GB free space. After doing the migration, I still have 10 GB free space.

(Note that you can then safely delete the iPhoto library afterwards, the file system is smart enough. See https://apple.stackexchange.com/a/182799/172153.)

  • You meant that after the migration from iPhoto to Photos, before you delete the iPhoto library, the amount of free space didn't seem to change. Don't you? – hectorpal Feb 26 '16 at 18:50
  • Correct. But also not after I deleted the iPhoto Library. – Wim Leers Dec 30 '17 at 0:02
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If you migrate the Library from your existing iPhoto external drive to a new Photos library in that same location, you will actually not increase the real storage space used by much at all.

What you will see - which might look disturbing, but isn't actually cause for concern - is that Get Info on a 200GB partition will show the file contents to be 270GB… more than the drive has space for!

This is because the Finder does not take the time to adjust for hard linked files & just takes the 'face value' of the reported size.
The real data size would be your original 135GB, plus perhaps a few needed for the new folder hierarchy & I would guess any thumbnail structures. Maybe several GB, but nowhere near the apparent 135GB that it will show in Get Info.

After that, deleting either library will reduce the apparent size but not the real size. Only when both libraries are deleted will the entire 135GB be discarded.

  • Yes, thanks. I know it's using hard links. (I'll edit the question accordingly). But wonder how does it behaves during migration. Maybe the caching index or other structures add an overhead. How much is that? – hectorpal Dec 14 '15 at 11:35
  • During migration it is busy creating the hard links… The overhead will be nothing like 'double the data' – Tetsujin Dec 14 '15 at 11:37
  • Yeap, the overhead wouldn't doubling the data. But different indexing systems have different overhead. That could be problematic when migrating in a volume with not much space. The question is how much is enough? – hectorpal Dec 14 '15 at 11:46
  • Take for example this discussion on the impact on migrating from FAT to NTFS. support.microsoft.com/en-us/kb/156560. That's a file system. But in this cases there is also indexing of the pictures. – hectorpal Dec 14 '15 at 11:47
  • Thanks for your comment and discussion. I've improved the question. – hectorpal Dec 14 '15 at 11:51

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