Cannot change ownership on some USB sticks?

[gordon]

@arQon

I don't remember where the cache is (IIRC it's a pair of files per device/FS ever mounted)

I'm getting deja vu from this comment! Remember this comment from May?

Caja - slow folder refresh

It's GVFS. It's under your home directory, specifically in ~/.local/share/gvfs-metadata. @arvindpmate, and after disabling all thumbnailing, delete the directory ~/.thumbnails in your home directory. This will get rid of existing thumbnails. (I deleted mine recently and discovered there were 25,864 thumbnails in that directory taking up a total of 36 MB of disk space!)

TL;DR: ~/.local/share/gvfs-metadata.