EOF during large file export after syncing filesystems

[rocodes]

Description

From freedomofpress/securedrop-workstation#1103 (comment)

I had one failure where send-to-usb returned EOF after reaching Syncing filesystems, but I've not been able to reproduce it in repeated testing with a ~500 MB file.

Steps to Reproduce

needs reproducing

Expected Behavior

export status returned

Actual Behavior

invalid status (needs repro)

Comments

May be able to fix without reproducing

[rocodes]

@cfm If you still have the sd-devices logs from the session where this occurred saved in sd-log, they would be helpful :)

[cfm]

Sorry, @rocodes; I didn't see this before wiping the system for 1.0.0-rc3. What I recall from yesterday is that there were only happy indications and no errors in send-to-usb's logs, and the actual export succeeded despite the EOF returned.