iPhoto stores photos in a single giant database blob, a .photolibrary file. And by giant, I mean for an average photo library that has been in use for several years, it may be 100GB or more.
Note: This will also apply to the new Photos app, with the file extension changing to .photoslibrary
Using this single file creates a single point of failure for losing your photos.
I have seen this file loss happen, with an entire year of photos gone.
Here are the issues:
- iPhoto using a single file to store all of your photos
- the .photolibrary file can grow very large, 100GB or more
- large files take a long time to backup, particularly if you are doing backup over wifi
- Time Machine canʼt do a full iPhoto Library backup when iPhoto is open
- large files that have been used for years may develop errors
So here's what happens:
- user has a single iPhoto database that has been in use for years (over 15,000 photos)
- user almost always has iPhoto open, meaning the library is rarely backed up by Time Machine
- the library is so large that it is slow to backup anyway
- eventually the library has a fatal error
- there is no recent version of the library to recover from backup
As a result, an entire year of photos are gone. (This is a real example.)
To mitigate this risk:
- do not use one single giant iPhoto library; make new ones e.g. a new one each year
- make sure you close iPhoto at least at night so the library can be backed-up overnight
- make sure you have a complete copy of the individual photo files also in the file system, also backed up
You can get individual photos out of iPhoto through export, through software that will extract the photo files, through manually browsing and copying from the library (see Where iPhoto Pictures are Located - Accessing the Picture Files and Originals), or by doing a separate download into the filesystem each time you copy files from your photo device (you can use Image Capture to download files into a directory, although by default it will dump them all without any folder structure).
UPDATE 2015-04-20: To access the folder containing all of the individual photos in the iPhoto .photolibrary, first open Pictures, then right-click on iPhoto Library and select "Show Package Contents" in the popup menu. You will then get a pseudodirectory display. All of the photos are contained in the directory Masters, organised according to the whims of iPhoto. This is a copyable directory, you can e.g. drag and drop the entire Masters directory to an external hard drive. (Note: You must leave the original Masters folder inside the package, otherwise your photo library will break.) ENDUPDATE
By having small libraries, ensuring that the libraries are backed up, and by having a completely separate copy of each individual photo file, you will have the best chance of preserving your photos.
Sidebar: I wouldn't have thought that it was possible either for the library not to be backed up for an entire year, or for the .photolibrary to error out in such a way that it disappeared unrecoverably (not even DiskWarrior could find it). But both of those were the case. Murphy's Law applies with a vengeance to computers.