Since you've been so kind not to provide enough info I'm gonna have to use the old crystal ball trick.
First of all I'm guessing that you use the VMware version of BT4b.
So if my sixth sense is correct the 'safely remove hardware' would be from devices switching (e.g. wireless card) between the HOST and GUEST OS.
And for the final act the all-time favourite............
And if this doesn't help please do send us the exact text.