Your external drive has hidden folders on it, and one is a second Trash
Copied 400 GB and the drive says 430 GB used. Where the difference goes, including the Trash that empties separately, and how to clear each part.
You copy files to an external drive, the drive reports more used space than the files you copied, and Finder shows nothing to account for the difference. Every Mac formatted drive carries hidden folders, and one of them is a Trash that empties separately from the one in your Dock.
See what is really on it
du -sh /Volumes/YourDrive/.* /Volumes/YourDrive/* 2>/dev/null | sort -h | tail -15The leading dot in the first pattern is what includes the hidden folders. Substitute your drive's name, which ls /Volumes will tell you.
The second Trash
Deleting a file from an external drive moves it into a .Trashes folder on that drive, not to your Mac's internal disk. The space is not freed until you empty the Trash while the drive is connected, and people routinely eject the drive before doing that.
sudo du -sh /Volumes/YourDrive/.Trashes 2>/dev/nullIf that number is large, connect the drive, empty the Trash from the Dock, and the space returns. This one cause explains most cases of a full drive with nothing visible on it.
The other hidden folders
| Folder | What it is | Remove it |
|---|---|---|
.Trashes | Deleted files from that volume | Empty the Trash while connected |
.Spotlight-V100 | Search index for the drive | Only via mdutil, if you never search it |
.fseventsd | Log of file changes, used by backups | Leave it. It is small and rebuilt |
.TemporaryItems | Interrupted copies | Safe to remove when nothing is copying |
.DS_Store | Finder view settings, one per folder | Harmless, tiny, endless |
If the drive is a Time Machine destination
Then the answer is different: Time Machine fills the drive on purpose, keeping backups until it needs the space. That is the design rather than a fault, and it is covered in how Time Machine uses disk, including how to remove old backups properly rather than by deleting folders.
The format matters too
A drive formatted as exFAT or FAT32 for Windows compatibility has two traps: FAT32 refuses any single file over 4 GB, which is a common surprise with video, and neither format supports the metadata macOS uses, so copies can be slower and less reliable. For a drive that will only ever be used with a Mac, APFS or Mac OS Extended is the better choice.
If you are moving files off the internal drive rather than tidying an archive, what moves safely to an external drive covers which folders tolerate it.
Common questions
Why does my external drive show less free space than expected?
Usually because deleted files are sitting in a hidden .Trashes folder on the drive itself, which only empties when you empty the Trash with the drive connected. Search index and event log folders account for a smaller share.
What is the .Trashes folder on an external drive?
It is that volume's own Trash. Files deleted from the drive move there rather than to your Mac's internal disk, and the space is not freed until the Trash is emptied while the drive is connected.
Can I delete .fseventsd from a drive?
You can, and macOS recreates it. It is a small log of file system changes used by backup and sync software, so removing it gains almost nothing and can make a backup do more work next time.
Should I format an external drive as exFAT or APFS?
APFS if the drive is only ever used with Macs, since it supports the metadata macOS relies on. exFAT only if you need Windows compatibility, and avoid FAT32 entirely for video, since it cannot store a single file larger than 4 GB.