external disk has always problems ejecting safely

Are you sure there is no program using it?

This happens to me when I leave final cut open after working on the drive or if I've emptied the trash while the drive is connected, the Mac will then clear off all the deleted files that I've ever deleted off the external drive and this can sometimes take a long time depending on how long its been since the last time I emptied trash while the hdd was connected.

using time machine?

 

Next time you eject it, quit all of your open applications one at a time until you figure out which one is holding the disk hostage. Then you'll have your answer. Based on my experience, Preview is often the culprit.

 

Next time it happens, open your "Activity Monitor.app" (Applications -> Utilities) and see if you can spot what is using the drive. If you find it, you can quit the app or process right from the Activity Monitor and then eject the drive. Sometimes though, nothing works. In that case, just reboot and remove the drive.

 

Reformat the drive.