What sort of media are you using? Does it load into a “clean” fcp?
I had a problem after the upgrade to 10.0.1 where for some reason it would not read my H624 files generated from a VIO POV.hd and input as linked files.
10.0.0 did read these files , and other makes of H264 did work. ( Motion would not handle them either.)
My computer just went into an endless loop when trying to access events (update) with these files.
I could not even run FCX if I had a drive connected that had events with these files.
It also would not import them directly, but had a similar loop crash.
Opening a project/events without these files was fine.
In the end I opened the events in 10.0.0 and transcoded all of my media to prores, to use with 10.0.1 .
Apple engineers acknowledged the problem on importing these files then 10.0.2 fixed the problem, perhaps 10.0.3 has problems with similar media?
Steve