Disk utility shows unattached external HDD

I have an external hard drive (WD Elements 1TB) that I use for Time Machine backups on my Mid-2015 Retina MacBook Pro 15" with macOS Sierra 10.12.3.

Lately, the drive doesn't get mounted any more when I plug it in. In Disk Utility, I see two instances of the drive when it is plugged in. When I unplug it, one of the instances disappears. The other one stays, even when I select "Eject" from the context menu. After a reboot, the "ghost instance" is gone, and the drive gets mounted again on plugin. But after one or two eject/unplug/plug-in cycles, I have the same problem again. Time Machine doesn't do backups to the drive unless it is fully mounted.

Is there any way to forcibly "eject" the "ghost" of the unattached drive?

As requested, here's the output of "diskutil list" and "gpt show":

$ diskutil list
/dev/disk0 (internal, physical):
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:      GUID_partition_scheme                        *1.0 TB     disk0
   1:                        EFI EFI                     209.7 MB   disk0s1
   2:          Apple_CoreStorage Macintosh HD            999.7 GB   disk0s2
   3:                 Apple_Boot Recovery HD             650.0 MB   disk0s3

/dev/disk1 (internal, virtual):
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:                            Macintosh HD           +999.3 GB   disk1
                                 Logical Volume on disk0s2
                                 2E277268-61D5-4149-B45C-39204E7C1CD8
                                 Unlocked Encrypted

/dev/disk2 (external, physical):
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:      GUID_partition_scheme                        *1.0 TB     disk2
   1:                        EFI EFI                     209.7 MB   disk2s1
   2:          Apple_CoreStorage TimeMachine BFD         999.9 GB   disk2s2
   3:                 Apple_Boot Boot OS X               134.2 MB   disk2s3

/dev/disk3 (external, virtual):
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:                            TimeMachine BFD        +999.5 GB   disk3
                                 Logical Volume TimeMachine BFD on disk2s2
                                 A7F8C874-D9AE-4818-98EE-C4D6F5DC610A
                                 Locked Encrypted

Offline
                                 Logical Volume TimeMachine BFD on disk2s2
                                 A7F8C874-D9AE-4818-98EE-C4D6F5DC610A
                                 Locked Encrypted

$ sudo gpt -r show disk2
gpt show: unable to open device 'disk2': Device not configured

$ sudo gpt -r show disk3
gpt show: unable to open device 'disk3': Resource busy

and two screenshots of Disk Utility:

External HDD attached:

External HDD not attached:

I seems found a work around to avoid reboot. After eject the Time Machine volume in the normal way, I open disk utility and eject the physical drive there, and wait untill the disk LED indicator shows that it is in sleeping mode (slow blinking) and then disconnect the drive. Now the physical drive is gone in disk utility, and I can close/sleep the MacBookPro. Next time when connect the drive, it will show up normally.

I wonder if this is because OS X sierra didn't handle the ejection process for encrypted drive properly.