Unmounting flash stick (problem)

I have difficulty unmounting/removing my flash stick. It's a Kingston Digital 64 GB Data Traveler SE9 G2 USB 3.0 Flash Drive.

On my Windows 10 machine, whenever I try to remove it, it always says that it can't until I stop some process, but never tells me what that is. However, if I close Chrome, Firefox, and sometimes Opera (yes, I'm running all of them, and not PortableApps), then, and only then, can I remove the flash stick.

To my knowledge, I am not doing anything that is browser based with my PortableApps--I'm just trying to synch files between my home and work PCs. So I'm mainly using FreeFileSync from the flash stick (PortableApps version).

Is there a way to guillotine all processes to the flash stick? Or another quick, otherwise non-disruptive way to remove the flash stick?

Thanks.

Phil