Version: 53.0.2875.41
OS: Chrome OS
Device: Minnie
(also reproduced with ToT Cyan)

What steps will reproduce the problem?
(1) Format an SD card or a USB flash with exFAT (e.g. by running mkfs.exfat on a developer Chromebook).
(2) Insert it to a Chromebook.
(3) On Files app, try to copy a file to it.

What is the expected output?
The file is copied.

What do you see instead?
An error is shown: "Copy operation failed. The file could not be modified."

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