[Unity 5]Problems about Lightmapping

I have a working on project works on 4.6.3, after Unity 5 released, I want to try and upgrade with 5, but I feel there are really too much problems on Unity 5, I tried to created to clean porject to make sure it is not the issue from the project on 4.6.3, the scene size is 300 X 300, the setup I used is try to make similar graphics on 4.6.3

then problem happen!

Failed writing to: 'C:/Users/Roger/AppData/LocalLow/Unity/Caches/GiCache/f8/f8fa93f003097cd9ad2063451442571c._comp_light.exr.tmp'.
'Compositing' job failed with error code: 6 ('Failed writing or importing the composited texture file.').

Same time, as you can see, the lightmap generated is 21.3MB, it is 5.3MB with same assets on 4.6.3.
Similar problem already keep for a long time since RC version.
I am really worry about what can I do for 5? does 5 only works with full realtime GI?

 

I have the same problem, an existing project (iOS mobile) 4.6.3 using dual light maps and it's now broken (very dark scene) in Unity 5 as existing light maps aren't being used.

How do I get the old light maps added to the project?