lsof

# This command lists all open files belonging to PID (process ID) 11925:
lsof -p 11925

# This command lists all open files belonging to processes owned by the user named "al":
lsof -u al

# This command lists files that are open in the directory specified, but it does not descend into sub-directories:
lsof +d '/Users/al'

# The next command lists files that are open in the directory specified, and also descends into sub-directories. Beware: this can take a very long time to run for large directory structures:
lsof +D '/Users/al'

# Find what is using a file or mountpoint
lsof +f -- /mnt/sda/automount/Media