When you plug in an external disk drive or a USB flash memory drive, OS X mounts them and makes them available to use, and they appear in Finder and on the desktop.
When you have finished with disks they should be ejected properly, which unmounts them. You’ll need to take care though, because files can be corrupted if you don’t follow the right procedure.
One of the main causes of problems is pulling out the plug when files are open. This leads to corrupted files and you might be unable to save work you have open. Files must be closed and applications quit before drives can be ejected safely. This is to ensure that all the data is written to the drive.
1 Do not unplug
Don’t pull out the USB flash drive or USB disk drive plug, or OS X will inform you that you have done it incorrectly. You will be told to eject the disk before disconnecting it.
2 Eject a disk
If there is an icon on the desktop, drag it to the Trash. It’s not deleted, it is ejected. You can right click disks in Finder or click the Eject button in the sidebar.
3 Eject partitions
If a disk is a partition on a drive, you are asked if you want to eject just this partition or all of them. Click Eject or Eject All. Keyboard shortcuts can do this too.
4 Avoid common problems
A frequent cause for the failure to eject a disk is because an application has a file open. Close all the files and quit all applications, then try ejecting the disk again.