Worried. iPhoto lost track of all full-sized images.

I'm on the latest everything, I believe: OS 10.10.2 and iPhoto 9.6.1. Running on a 1-year old Retina MacBook Pro, on an external drive. Very large iPhoto library (600+ GB), but no major issues to date.

I've recently purchased an iPhone 6. Not sure if that's a factor. Have been importing photos from it since purchasing. Just via cable... not iCloud.

When I review my iPhoto library in thumbnail mode, all looks fine. When I try to view an image full size, email an image, or export an image, I get a big exclamation mark. I saw an error at one point (during export of images attempt), that basically said file not found in original location. The directory structure suggested a folder name called "Masters".

When I review the contents of the iphoto library, I only have "Old Masters" and "Old Previews". That said, within those folders, I'm not seeing all the appropriate dates. So, I'm worried on a couple levels. Not sure what's going on. Import took the right amount of time, as if it was writing the full-size files to disk, from my iPhone 6, but I don't see the files there.

There are still plenty of files in the folder, including plenty of recent ones. None show up in full-size view in iPhoto, logically... since it appears that the app is looking for a folder called "Masters", not "Old Masters". However, thumbnail or preview image are showing up. So, it's not totally logical.

My instinct would be to just rename the folders, removing "Old" but I'm worried that I'll break something further.

Sample folder structure is included. I see aliases to "Originals" and "Previews" but clicking on them doesn't do anything.

Where do I go from here?

Thanks!

Matt