Why force eject hard drive every time?

I have a MacBook Pro laptop. When it's on my desk, I have a bunch of things, including a backup drive, plugged into it.

When I want to take my laptop away from my desk, I have to unplug these things. But, before I unplug the FireWire cable to the backup drive, I always first eject the drive. Sometimes I have to wait for Time Machine to wind down, then the eject happens. This is the way it has been working for years.

Then last November I upgraded from 10.6.8 to 10.11.2. Now, every time -- EVERY TIME -- I push the tiny icon in a Finder window to eject the backup drive, up pops an alert:

"Force Eject" always works and is now part of my routine. But it doesn't seem right. What's causing this extra step?

Only Time Machine should be using that disk. With Finder, I see nothing that isn't part of Time Machine's complicated data base.

I haven't received any messages from Time Machine; a cursory inspection of the backup looks good.

I tried typing lsof into Terminal, and didn't see any strange open files.:

$ lsof | grep Volumes

FolderAct 289 gil 5r DIR 1,2 170 13353610 /Volumes

I even opened a file on my backup drive, just to test that my lsof command was working:

$ lsof | grep Volumes

Finder 269 gil 24r REG 1,8 149158 692699 /Volumes/Orange Backup of MBProHD/Backups.backupdb/MBPro/2016-01-08-090714/MBProHD/Install Log

FolderAct 289 gil 5r DIR 1,2 170 13353610 /Volumes

I have an easy workaround, but still this anomaly bugs me.

Any suggestions?

--Gil