The following forum message was posted by at http://sourceforge.net/projects/s3tools/forums/forum/618865/topic/5588877:
Is the file very large? Or, was it uploaded in a multipart upload?
Multipart uploads on S3 don't get normal MD5 ETags. Instead they get an incorrect
ETag with a dash and a number on the end.
From what I've read, multipart uploads to S3 are only necessary on files larger
than 5GB, so not everybody comes up against this problem.
If your ETag at S3 has a dash and a number on the end, then you're probably
having the same problem. Luckily, there is a simple fix: Move the S3 object
(it doesn't matter where); this will cause S3 to recalculate the ETag and produce
a correct MD5 value.
The simplest way to do this is to move the object to its existing location:
[code]s3cmd mv s3://BUCKET1/OBJECT1 s3://BUCKET1/OBJECT1[/code]
Try that; it might just fix your problem.