converting stp. file to sldprt.

When I open an stp. file with multiple bodies, and try to save it as an sldasm., SW gives the following:

So I click save all,  but when I go to open the assembly document again, it seems that there is nothing in any of the part files that were saved. While opening, an error comes up for each one of the part files that says "An error was encountered while trying to open the file." I click OK for each one, so SW opens up an empty document.  If I try to open the individual parts, an error says, "An attempt was made to access an unnamed file past its end."  What am I doing wrong? Or are the files corrupt?