Hello everyone, and especially @ncw :
The latest "rclone sync" (with rclone's latest beta, v1.36-98-g7ee3cfd7β) has finished here, and this time there were 70 files copied from that totally quiescent (as in "read-only mounted") file system :-/
So, whatever the problem is, it persists with the latest rclone.
For more detail, here are all the "-v -v" log lines that mention one of those 70 files:
2017/05/18 12:36:18 INFO : REDACTED/REDACTED/REDACTED/REDACTED/REDACTED/REDACTED/REDACTED/REDACTED/REDACTED/REDACTED.info: Copied (new)
2017/05/19 10:12:27 INFO : REDACTED/REDACTED/REDACTED/REDACTED/REDACTED/REDACTED/REDACTED/REDACTED/REDACTED/REDACTED.info: Moved (server side)
2017/05/19 10:12:27 INFO : REDACTED/REDACTED/REDACTED/REDACTED/REDACTED/REDACTED/REDACTED/REDACTED/REDACTED/REDACTED.info: Moved into backup dir
Looking around each of those 3 lines, I see nothing out of ordinary (just "Reading", "Finished reading", "Size and modification time the same" and similar messages around the first line above, and the same messages for other files in the same situation, in the case of the last 2 lines above).
EDIT: now I think I understand why these files are being copied over and over; it's not because a "rclone sync" erroneously decides the file has changed in the local filesystem and needs to be copied, it's because a previous "rclone sync" erroneously decided to remove that file from the remote (by moving it server-side to the backup-dir, see the last two lines above). So this unduly removal (and not the copying per se) is the issue that needs to be addressed.