MacBook Air running Yosemite won't shut down when external HDD connected My MacBook Air won't shut down when an external hard drive is connected. The screen goes black, and the mouse pointer is still there. I think this started happening when I upgrade to Yosemite. Any suggestions as to how I might fix this?
Are you sure something is not still running...or something is preventing the disk from unmounting.
If there's a grey circle -- then it might be running fsck on the disk...
Now, another thread suggested:

Start Up in Verbose Mode using Command-V
Then issue: sudo shutdown -h now
Also, more /var/log/system.log

This should show you what's occurring as it shuts down.
The sudo shutdown -h now is equivalent to the Shut down menu option.
Also, try going into Disk Utility and manually click on the Unmount icon.  I would just want to see if it will unmount normally.
If it doesn't, then, I'm thinking SOMETHING is still using  the disk..