Each disk/volume has its own trash, so we first have to determine which trash the 40GB ended up in, and if the original your were copying over still exists on the internal of the Mac. The latter is easy to answer just by looking, the second by simply disconnecting the external first then looking in the Trash of the Mac. If it's not there, it's in the external's Trash.
First, see if the trash will empty this way:
- Press and hold the mouse button on the Trash icon in the Dock. The context menu for Trash will display.
- Press and hold the Option key or the Shift-Option keyboard combination,
- Select Empty Trash from the context menu for Trash.
- Release the keys pressed and held in step 2.
If you still have the original you were copying over on the internal of the Mac--you don't want to do this unless you do--and what your were copying over resides now in the Mac's Trash, then copy/paste only the following into Terminal in Utilities, which will nuke the Mac's Trash. A typo can be disastrous, so only copy/paste.
sudo rm -rf ~/.Trash
Hit return and you will be prompted for your admin password, which won't be displayed in any form. You will also get a scary warning about using the sudo prefix. Hit return again and the entire Trash of the Mac, including the Trash folder, will be nuked. It will take some time to empty the 40GB, so wait until you get a new $ sign prompt, which will indicate it's finished. When you do just close the window. To restore the Mac's trash folder, not the contents, just log out and back in.
If the 40GB is in the trash of the external, then you will have to enter the following. And here, since I don't know the name of the external volume, you will have to be extremely careful. If there are any spaces in the name of the external, it must be entered, as in the example below, in quotes. If not, then no quotes. Note: there is a space between sudo rm and between rm and -rf. There is no space between / (the forward slash) and Volumes. This is critical. Trashes here is plural. When entering the name of the external, pay attention to case. Except for the possibility of spaces in the name of the external, there are no more spaces in this command.
sudo rm -rf /Volumes/"name of external volume"/.Trashes
