1. Read the message first
When emptying fails, macOS says why: the item is locked, in use, or you do not have permission. That sentence decides the fix, so do not skip it and reach for a command from a forum. Apple’s storage guidance also explains why this matters for space: a file moved to the Trash keeps its storage until the Trash is emptied.
Open the Trash and look at what is there. A single stubborn item often blocks the whole operation, and it is usually recognisable: an installer that is still mounted, a document open in an app, a virtual machine, or a folder copied from an external disk.
2. Locked items: unlock with Get Info
Apple’s guide gives the step: select the item, choose File → Get Info or press Command-I, and deselect the Locked checkbox. If the checkbox is greyed out, click the lock at the bottom of the window and enter an administrator name and password first. Then empty the Trash again.
Files copied from another Mac or restored from a backup can arrive locked. Unlocking them changes nothing else about the file, so it is safe to do before you decide whether to delete.
3. Items in use: quit the app, then try again
A file that an app has open cannot be removed until the app lets go. Quit the app that created or opened it and empty the Trash again. When you are not sure which app it is, close everything, then log out and back in, or restart, and empty the Trash before opening anything else. This is our practical order; it clears the lock without touching the file system directly.
Modern macOS has no Secure Empty Trash, and the commands people paste from forums bypass the Trash rather than fixing the lock. Removing files outside the Trash removes the ability to put them back, and it does not release a file that a running process still holds.
4. Delete a single stubborn item
Apple documents a per-item route: Control-click the item in the Trash and choose Delete Immediately. It removes that item permanently and leaves the rest of the Trash where it is, so use File → Put Back on anything you want to keep first.
If Delete Immediately also fails with an in-use message, the lock is real and the app or process behind it still needs to quit. Restart and try again before anything else.
5. Trash on external disks, and what still counts
Each disk keeps its own Trash. Items you deleted from an external drive appear in the Trash only while that drive is connected, and the space they use is on that drive, not your Mac. Connect the disk, then empty the Trash; ejecting it does not free anything. A backup disk used by Time Machine is managed by Time Machine, and its backups are not meant to be deleted through the Trash at all.
After the Trash empties, check System Settings → General → Storage. If the available figure has not moved, the snapshots and storage-not-updating guides explain purgeable space and delayed figures. ClearDisk’s own removal is Trash-first for exactly this reason: files go where you can still put them back, and the space returns when you empty the Trash yourself.
References & further reading
These guides combine the linked official guidance with ClearDisk’s documented behavior. Your files and app settings determine the right next step.