Force eject a device

Solution 1:

diskutil eject does not accept force (see man diskutil) and if the disk is in use may fail with "Volume timed out while waiting to eject"

 eject device
            Eject a disk.  Media will become offline for the purposes of
            being a data store for file systems or being a member of con-
            structs such as software RAID or direct data.  Additionally,
            removable media will become eligible for safe manual removal;
            automatically-removable media will begin its physical (motor-
            ized) eject sequence.

If you want to eject the disk (which will mean it is not available to be mounted until you reboot or unplug/reinsert it) then it would be you should unmount any filesystems first for example using diskutil unmountDisk

 unmountDisk | umountDisk [force] device
            Unmount an entire disk (all volumes).  Force will force-
            unmount the volumes (less kind to any open files; see also umount (8)).

Assuming you want disk2 this would mean running in terminal either

diskutil unmountDisk /dev/disk2

or (if you want to force it with associated risks)

diskutil unmountDisk force /dev/disk2

followed by

diskutil eject /dev/disk2