very long time to safely remove externak disk

Greetings
this is new situation for me.
It never took so long to safely remove external disk.
I'm using 1TB WD my passport ultra with over 700MB free to copy 24GB (backup) and it takes too long to eject it. The external disk is directly connected to a USB3 socket according to the motherboard (Gigabyte) manual.
It used to be much quicker.
I also tried it with a 2TB WD elements with 1.8FB free.
what could cause such slow response? do I really need to wait? can I disconnect the usb cable without damaging the files copied to the external disk?
thank you
I hope drivers & hardware is the right place for this post

When you click on "safely remove" do you see the external drive activity light constantly active? Make sure you have Quick Removal set in Device Manager->Disk Drives for the drive.

thanks for your prompt answer
yes, I have quick removal radio button checked (it is the default)
The light blinks a little and stops. When I try to eject it blinks a little and stops again.
Every trial to eject opens a "problem ejecting USB mass storage device" window informing that the device is currently in use, even if I did not copy anything!
On the disk all files I copied are there with the right size but of course it does not prove that they were copied correctly.
Since this is a new situation and I hadn't changed anything and external disks are quite some time in use, I'm worried.

Berton,
I'm using Kaspersky antivirus for years and I wait till the tray icon stops moving before I try to eject.
In any case this is new. Until two weeks ago I didn't have any problems with my external disks
thank you