Library Upgrader unsuccesful

I am trying to upgrade a iPhoto library that is saved on an external hard drive, which is not formatted for Mac. I would like to open it with iPhoto on my brand new MacBook Air OS X Yosemite.

I don't remember the version of my iPhoto library, but I know that it was built on a OS X 10.6.

Library Upgrader first tells me that it detected errors in my library and suggest to repair them, which I accepted. The repair process is very fast and I click on Upgrade.

At the end of the (also very fast) upgrading process, I get a message of successful completion of the upgrade, inviting me to open iPhoto. However, iPhoto shows again the same failure message that the library needs to be upgraded... Any hint to help me?