You can try the following steps:
1. Disconnect all peripheral devices (for example, hard disk or printer).
2. Make sure that the Mac has installed all available software updates. (Important: Please back up your Mac before updating. )
3. Try to force all background applications to quit, and then restart the Mac.
4. Try to reset the nonvolatile random access memory (NVRAM).
5. Try to create a new administrator account and log in with the newly created administrator account to see if the problem still exists:
Select apple menu (_) > system preferences > users and groups in the upper left corner of the screen. Click the lock icon in the lower left corner and enter an existing administrator name and password to unlock it. Click Add (+) below the list of users and select Administrator from the New Account pop-up menu. Enter the full name and password of the new administrator user, and then click Create User.
6. If possible, try copying other files or dragging them to other folders to see if the problem still occurs.