Split second after copying files from usb to folder in finder the files disappear

I have this problem in mac Lion whereas when you copy files from a usb harddrive or flshdrive it show that the files are being copied and if you watch the folder when the copy is in progress it shows the files as greyed out which is correct, but a split second after the copy process is finished the files just disappear without an error or any messages its not hidden or anything. If i do the copy from the terminal it copies the files without any issues. Is this maybe a bug in Finder??? sometimes when i reboot then it works