media offline in final cut pro

I´ve overwrote my film project on my Mac Mini, with new edited files from my external harddisk. When I open the project in Final Cut pro, some clips in the timeline are offline. Still I can play it as normal, and export it as normal. When I click on the clips / go reconnect media (it finds the source automatically), it goes back to online..., but only for some time.

Does anyone have a clue what this is?
If I play it from my external harddisk everything is fine...


Kristine