Just a by the way, I had one N900 that I could not flash for more than a year with a similar error. Following the advice here, I tried it with three different operating systems on four different computers and with three different USB cables. Nothing. I put the phone in the drawer and gave up on it.

Then I upgraded my laptop and after a few months remembered about the N900. I tried it and, hey presto, it worked!

The upshot is that I can confirm, sometimes it just does not work with some computers but works with others.