Chrome Version: M71
OS: Chrome

What steps will reproduce the problem?
1. Create and format a partition using ext4 onto a USB drive
2. Plug drive into Chromebook
3. Copy file using file manager

What is the expected result?
File was copied.

What happens instead?
File not copied. "Copy operation failed. The file could not be modified." error message appears, which doesn't give any information on what's going on.

What appears to be happening is that because ext4 is a native linux filesystem, the permissions are being respected. Browsing in a terminal shows the mount directory being root:root ownership and 0755 permissions, which will prevent a regular user from creating any files.

According to https://support.google.com/chromebook/answer/183093?hl=en, we don't technically support ext2/3/4, but we it is supported and it should work reasonably, even if it is best-effort.