Backing-up your files and your system is important, but always be careful when inserting someone else's flash or USB drive into your computer. External drives can be filled with malware, and all it takes is for one well-placed "left behind" drive to infect an entire network. The bottom line: If it's not your device, don't use it. Scan your device regularly for viruses and other malicious programs to make sure that you don't infect any other machines.