I don't own a Mac myself I take that back. It is really old , but I have friends and relatives who do. This problem may come up and I'll know what to expect.
Cannot delete locked files on external volumes
This happens on the PC from time to time especially when working with large files. I posted about it on our blog see link below. Joshua - that's an interesting and handy! Windows app!
I haven't seen an equivalent app on the Mac, even though there have been a few times very rare, though when I tried to delete a file or eject a volume, only to have a window appear saying "This file [or disk] is in use. Please quit the application using it and try again.
How to empty trash on a Mac
Dealing with Locked Files on a Mac. This has the effect of protecting your folders and the files they contain with a password.
You can password-protect folders on macOS using the built-in Disk Utility application or a third-party tool called Encrypto. You can password-protect the contents of a folder by copying them to a password-protected disk image. This will create a new dmg file which will be password-protected. The original folder will remain untouched and can be deleted once the contents of the disk image are confirmed.
Both formats are sufficiently secure for protecting your data, so the faster bit AES is typically recommended. Enter a password when prompted. This is the password you will enter to unlock the disk image.
It should not be the same as your Mac password. Make sure it is sufficiently difficult to guess. You can encrypt your entire hard drive, encrypt an external drive, or just create an encrypted container for your most important files.
When you enable FileVault, your files are stored on your hard drive in an encrypted, seemingly scrambled format. You can choose which user accounts have the ability to unlock your disk.
Your drive will be locked again when you shut down your Mac. This allows you to regain access to the drive if you forget the username and password for the local account on your Mac. This can take days, so consider keeping your Mac awake overnight. With macOS you can also encrypt entire external drives.
[Solved] How to encrypt external hard drive for Mac/Windows usage?
The contents of the drive will be encrypted with a passphrase you choose, and no one will be able to access them without that passphrase. To encrypt a drive, simply open the Finder and connect the drive to your Mac. The disk will be encrypted once you enter your password of choice—be sure to use a secure one!