Ok, it sucks when it happens but I may be able to help. Right now, I'm in the process of writing a program that will repair corrupt NS1 files!
The testing so far has gone well but I only have 3 corrupted files. I need more files that have been corrupted naturally, so that I can code the program correctly.
So, here's what I need YOU to do. Zip up your corrupt Ns1 files. Each message should be under 3Mb lest my ISP give me a nasty gram. Then send them to Beakmyn <at> Frontiernet <dot> Net
I will test them with my program and if successful, I'll email you back the fixed ns1 file.
Just to give you an idea of the progress so far:
I've found one corruption that is automagically repaired. I'm still working on writing the records back out into Ns1 format. Not as easy as you'd think since I'm using <cough> Visual Basic <cough> So, it may be a few weeks before you get your file back.
Once tested the program will be released for your enjoyment.