HELP! I had nearly 50,000 photos in my iPhoto library and then upgraded to 10.11.2 version of El Capitan. Now only 794 have populated into the Pictures folder including many recent iCloud shots. But nothing from before 2013 (there were thousands). I can't find/import anything that's not here - please help, i'm very concerned the library is gone.
Sorry to hear about the missing photos, and based on the last entry, I gather they've yet been found. If the photos were somehow deleted, they would/will remain accessible indefinitely until their previous space/location get overwritten (not the most eloquently written). So, it is key that you not use that drive, which may be a moot point here, several months after the fact. If that means pulling and shelving the drive so you can maintain productivity while keeping open recovery as an option, then pull it.
If you don't mind sacrificing some of the organizational aspects, like tags and albums, this may work. Download and install InVisible from NorthernSpySoftware [http://www.northernspysoftware.com/download/InVisible_1.3.dmg]. Run it, if only for this project, to reveal all hidden files (i.e.: your missing photos). From the desktop/finder hit the f key while holding the command key to bring up a Searching "This Mac" window. Toward the top left corner of said window, use the second drop-down menu to select Image. So now your search criteria read Kind is image. Make sure the right above the drop downs it says, "Search" This Mac".
This should produce a substantial number of images. Granted, this includes everything from random graphic elements to Word clipart to web history thumbnails, most of which is worthless. This should give you some insight into the location of the missing library.
For most, the default view settings are list. Try changing that to icon, by holding down command key and tapping 1, or by clicking the View Menu while having the Search window active/selected and going down to the first choice as icons. While in that same menu, come on down to Arrange By