Ok, let's start at the top. First of all, your analysis about the file is correct, but it won't help you.
mscorlib.dll is the core library of the .NET framework. It is where the FileNotFoundException is implemented. The error message does not indicate that there is an issue with mscorlib.dll.
The FileNotFoundException is thrown because the program you were using (the migration wizard) tried to access that file that simply did not exist.
However, at this point I am uncertain if that is even an issue here, as the log simply goes on. So either, it ignored the error (which leads to the following errors) or it is simply non-critical. Nevertheless, let's take a brief look.
Stage 1
At the start of your log, 3 exceptions are being thrown. All stack traces and exception descriptions sound like the wizard is enumerating your Windows Server Backup tasks in task scheduler (SBS uses Task Scheduler to set up Backup tasks). During this process of parsing the backup tasks, it tries to get access to a folder (possibly the backup target location) and that folder does not exist. Thus, the error in the log.
The wizard treats these exception as non-critical, which can be deduced from the fact that it kept writing to the log and this line, that indicates that an unknown state is an expected outcome of the process:
[1516] 110315.190857.6216: Storage: Backup Task State: Unknown
So, an error happened, and the state could not be determined. Let's move on.
Stage 3
This is where the next issue happens. The final, fatal one that will cause the wizard to abort.
The key element of this error message is
WMI error occurred while accessing drive
The wizard will try to get some information on your drive and it will fail at some point.
That error message will yield you much better results on the web. But I would urge you to do the search yourself and judge which results are helpful to you (possibly, your target disk is a dynamic disk)
Sorry that I couldn't offer you a direct solution, but I hope my analysis will help you in solving the issue. It would be appreciated if you report back and write your own answer if you're able to solve the problem :)