Since you said "fixmbr", I'll assume this is a Windows box. If you have the boot drives mirrored in Windows (vs. using the disk controller), there are a couple little "gotchas". The best thing to do is test it right after you load the OS and get it mirrored (by unplugging the primary drive and trying to boot).
Here's a Microsoft KB article that might help.
http://support.microsoft.com/kb/167045
Also, you need a boot.ini that points to the secondary drive. This can be done with a floppy boot disk.
http://support.microsoft.com/kb/311073/en-us
HTH, Rob