What usually holds it
External drives and SSDs are often held by backup programs and sync tools. If a backup is running, wait for it to finish. Pulling it out mid-write breaks that file.
How to eject it
- Close the Explorer window that opened the drive.
- Close the programs that have files on it open.
- Use LetGoFile to see whether anything is still holding it.
- Press Safely Remove again.
Get LetGoFile, look at the drive, and it tells you the name and PID of whatever is holding it.