The iMac OS is Yosemite. I have 500 mp3 files on an external drive (ExFat). If I move the drive to a windows machine, I see 500 files in Explorer. I then move the drive back to the iMac and PLAY file XYZ.mp3 The iMac still shows 500 files in Finder. If I turn on hidden files the Mac still shows 500 mp3s I then move the drive back to the windows machine and it shows 501 mp3 files. The extra file is the one that I played on the Mac and the file name is ._XYZ.mp3 and the size if 4KB It's an annoyance because every so often, I copy the music library to a USB drive for the car. The cars sees those extra files as valid mp3 files and wants to play then so unless I'm in shuffle mode, it plays every song twice. I'm not imagining this. Aside from using a windows machine and deleting the extra files every time I create an export for the car, is there any way to eliminate them on the Mac or prevent them from getting created in the first place.