- How do I list all unsynced/unwritten files on a usb-stick/sd-card or other removable-media?
Use lsof to list open files then use grep to filter the results to your mountpoint.
- Can I be certain all files are synced after I run sync?
If sync returns successfully then at that moment all files are synced. An unmount (or "eject" in the GUI) calls sync in the background before actually unmounting.