redsnow crashes mac

[mrporridge]

I'm jailbreaking iphone 4, 4.3.5
using redsnow 0.9.8b4

I get into DFU, its starts the jailbreak, then it crashes (I get a message saying that I need to restart my macbook, and my phone goes white)
I then fix with tinyUmb

Any ideas as to why it crashes?
I've read about changing CPU, is this necessary for Mac?

thanks