I experienced no problem with disc ejection under 10.11.1, however I installed 10.11.2 on December 17th, and this is my log to myself about what happened on completion of my first full backup to an external drive (a Samsung partitioned into 5 'cycles') after that installation...
Full Backup to Cycle 5 (bootable)
24.66 GB
About 560,031 files
(Sophos scanning switched off)
SuperDuper! finished as normal.
However, this backup 'failed': after completion unable to eject external disc using (1) keyboard eject button or (2) Disk Utility's eject menu option or (3) Disk Utility's eject button.
No response from (1).
Method (2) ejects all partitions (Cycles 1, 2, 3 and 4) but not Cycle 5. No reason given.
Method (3) returns 'Unable to eject Cycle 5 because the device is currently being used'.
No process initiated by me is using this backup disc.
A reboot (logging out and switching off), leaving the external drive connected, does not cause the drive to be re-mounted.
Physically disconnecting and re-connecting the drive after a reboot causes the drive to be mounted as normal, whereupon it can be ejected by method (2) above. Method (1) still has no effect.
Although the state of the contents of Cycle 5 remains unknown, I have no reason to think it has been corrupted.
I had no problem with El Capitan 10.11.1 but the very first time I tried to eject an external disc after installing 10.11.2 this occurred.
What diagnostics can be used under these circumstances to identify the process claiming to be still using the external disc?