peter yes but there is a chance he might have 1 or 2 files amongst the 1000 files he is trying to import that ST3 doasnt accepts of likes.
Perhaps if he gets the first six song, he should double check the seventh song to see if it is in any way different, and perhaps dont select the seventh song as an import and see how many more he can import before it stops again.
I am so sure i had this problem years ago, and my work around was to eliminate the culprit songs which were few and import the no problem majority, then i singly fixed the problem songs by importing them to a DAW and exporting them again then they loaded to ST3 ok.
I have experianced this through the years of using all different gear.
.mid loads in to my yamaha keyboard but .midi does not yet both are supported formats on other systems, so go figure.
I am just suggesting try to load all .mp3 files first then .wav and so on , i am eliminating possible causes to a problem.If that doasnt work then its something else.
Hopefully the problem songs are few and you get the majority loaded quickly in to ST3.