Mike,
I am sorry that I can't be more helpful but I have not been able to find a concrete answer to your question. The installation technology used in CVI 5.5 and before is from a third party that has since stopped making it. It is very hard to get any support.
Here are some suggestions:
Make sure the floppies are good and not write protected.
Make sure you are not pulling from someplace strange (subst drive, networked drive).
Make sure that the disk you are building from has plenty of space, but not more than 4 GB of free space (this caused problems with the old technology).
Other than that I can not find anything about error 551 on any website.
Jeff
NI