Recover Missing iPhoto File

I recently migrated from an old Mac Mini to a new Mac Mini running Yosemite 10.10.4 and now I can't find my iPhoto file. When I click on iPhoto, it's empty. The new "Photos" file only has the photos from my iPhone and iPad. I have an external hard drive attached, but it shows the same thing. I searched Time Machine, same thing. My one saving grace is that I have another external hard drive that probably has my iPhoto file, but I'm afraid to attach it for fear that it will "update" to the new "empty" file before I get a chance to retrieve it.

Can I turn off the update feature somewhere before attaching this other external hard drive? My husband always handled my "technical" things, but unfortunately he passed away a month ago. Please help me . . . . . . Most appreciated!

One other question . . . If I get this to work, will the iPhoto file automatically migrate into the new Photos file or will I have to do something to get it there?

Thanks for your consideration . . . .