The message about what an external drive (flash, HDD) can be removed actually appears much earlier than he actually released and unmounted.

[Andrey Skrypnik]

1. Linux Mint 17.3 - Rosa, Cinnamon 64-bit

2. I copied large files to an external drive. I click on the desktop on an external drive - extract. The message that the disk can be removed, but in fact continues to write files on the disk according to the indicator. But at the same time Nemo shows that copying is complete.

3. Remove the disc and the data is written partially lost. And suddenly there appears a message that could not unmount disk. Extraction can be carried out by any means, but the situation is similar. In fact, the ability to extract can be traced only disklet "Drives manager v.1.6": it changes the image first, and then when the external drive will disappear from the disklet it can be safely removed.

4. Probably not synchronize the file systems, similar to sync team. As an option - you need to run automatically Sync command before extracting.
Eventually:
The message about what an external drive (flash CDM) can be removed actually appears much earlier than he actually cleared and unmounted.
I expect that the message about the possibility of extracting the external drive should be appearing at the end of full time working with him and synchronization of file systems. To avoid losing files and get safe extracting.

5. The situation appears constantly with any external hard drives.

~ $ lspci | grep USB
00:12.0 USB controller: Advanced Micro Devices, Inc. [AMD/ATI] SB7x0/SB8x0/SB9x0 USB OHCI0 Controller
00:12.2 USB controller: Advanced Micro Devices, Inc. [AMD/ATI] SB7x0/SB8x0/SB9x0 USB EHCI Controller
00:13.0 USB controller: Advanced Micro Devices, Inc. [AMD/ATI] SB7x0/SB8x0/SB9x0 USB OHCI0 Controller
00:13.2 USB controller: Advanced Micro Devices, Inc. [AMD/ATI] SB7x0/SB8x0/SB9x0 USB EHCI Controller
00:14.5 USB controller: Advanced Micro Devices, Inc. [AMD/ATI] SB7x0/SB8x0/SB9x0 USB OHCI2 Controller
00:16.0 USB controller: Advanced Micro Devices, Inc. [AMD/ATI] SB7x0/SB8x0/SB9x0 USB OHCI0 Controller
00:16.2 USB controller: Advanced Micro Devices, Inc. [AMD/ATI] SB7x0/SB8x0/SB9x0 USB EHCI Controller
03:00.0 USB controller: ASMedia Technology Inc. ASM1042 SuperSpeed USB Host Controller
04:00.0 USB controller: ASMedia Technology Inc. ASM1042 SuperSpeed USB Host Controller