Not exactly related, but I wanted to add that just a few days ago Syncthing also helped diagnose a dying HDD.
Multiple errors like the one below suddenly popped up,
Fatal error: gdwva-7bfwn WithNeedTruncated(XXXXX): leveldb/table: corruption on data-block (pos=2014954): checksum mismatch, want=0xffffffff got=0x6170dc89 [file=001359.ldb]
and sure enough, we checked the HDD and the thing was toasted.
Yet again, Syncthing was the first to report the problems, as the OS and the rest of the software seemed to still work fine. The disk is now going through its last zero-format before being thrown away (which is taking forever due to all the write errors).