Open Terminal Enter the following: diskutil list Check which disknumber the USB-stick has. (disk0 or disk1 or disk2 etc) Enter the following: diskutil unmountDisk /dev/Disk# (Disk# = disk0 or disk 1 etc – see step 3) Open Disk Utility Click on the USB-stick Open the Erase-tab Choose the format  you want (Mac OS Extended(macosx)/ FAT [...]