Lightroom :: Save A Copy Of Raw Files For Security

I am having an ongoing problem with Lightroom 3.6. Once I have loaded the photos into Lightroom and decide to save a copy of the raw files for security, the files in Lightroom are "Lost" and I have to delete them all out of Lightroom and from the harddrive where they still are and reimport from the external harddrive. I am sure this is not normal. I would like to backup my photos especially while travelling but have resorted to "risking" it because of this issue. It happened once and I thought I must have done something daft but I was recently travelling again and exactly the same issue reacurred. I was travelling with a professional photographer who uses Lightroom all the time and he did not understand what was going on - he had never had the problem. The only difference was that I use a PC and he uses APPLE. I am very anxious to address this problem because I think I am tempting fate not backing up my photos. I am using a DELL XPS L502X Direct Base 15 laptop with i7; 8GB dual-channel; 750 GB 7200RPM hard drive and is less than 12 months old and I am using Lightroom 3.6 64 bit.