Chrome OS Version: 54.0.2806.0

Steps To Reproduce:
(1) Connect an external, unformatted disk. (e.g. a USB drive with all partitions removed)
(2) Open Files app. See the media appears as "USB Drive" on the left pane.
(3) Drag and drop files from Google Drive or Downloads folder to "USB Drive" while pressing a [shift] key. (file move operation)
   [WARNING: this operation may delete the files]

Expected Result:
- Files are not written to the USB drive. (because it's not writable)
- The files stays in the original directory.
- An error message pops up.

Actual Result:
- Files are not written to the USB drive.
- The files in the original place are deleted.
- No error messages pop up.

How frequently does this problem reproduce? (Always, sometimes, hard to
reproduce?)
Always.

What is the impact to the user, and is there a workaround? If so, what is
it?
Users may unintentionally delete files when attempting to move them to a USB drive.

Please provide any additional information below. Attach a screen shot or
log if possible.

This is also reproducible by using ctrl-x + ctrl-v. In this case, ctrl-v should be pressed right after clicking "USB Drive" in the left pane of Files app. You cannot paste files after clicking the right pane.

The same issue happens when copying files instead of moving except that the original files are retained.