My USB flash drive says it has 15.7GB of space used but inside all the folders are empty

• try a data recovery tool/file system scanning to see whether the file system is just corrupted
• open up the drive itself (most drives are snap lock designs. Easy to pry open using a sharp knife). Look for physical damage. In this case, my guess is that it's possible that the first chunk of the flash memory is unaffected but the chunk which actually holds the data is in trouble. Look for a gap between parts that should be in contact. It may only take a bit of bending to get back access to your files (had this problem before. Used a soldering iron to weld back relevant points)
• I've seen something like this with my one of my optical drives. Basically, anything it wrote would show up as zero sized files after written. It was only when I updated the firmware would things act normally. Try on a newer machine
• power problem. There is a malfunction electronically somewhere (on the drive itself or on your system). Power issues can result in a ll sorts of strange anomalies including possibly this
• use a different Operating System (Knoppix, a Live Linux Distribution is probably best suited to this) to see whether it is specific to it. Had something similar before where a NTFS filesystem would show up normally under Linux but corrupt under Windows

This is normal. An "empty" NTFS filesystem has a number of internal files such as the master file table ("$MFT"), the log file used for filesystem recovery ("$LogFile"), the volume descriptor file ("$Volume"), and so on.

The largest one is probably the cluster allocation bitmap ("$Bitmap") that keeps track of what space is used and what space is free. This is pre-allocated when the filesystem is created.