I would recommend just using the AWS Cli for windows instead of powershell

Uploading 650MB chunks using S3-WriteObject. Didn’t want to choke link during day so CTRL-C’d to exit the powershell script and found in the morning that the upload had happened any way.

The Amazon docs may have the answer: http://docs.aws.amazon.com/AmazonS3/latest/dev/uploadobjusingmpu.html Note what it says about multi-part uploads

“Once you initiate a multipart upload there is no expiry; you must explicitly complete or abort the multipart upload.” — Amazon Docs

And more:

Using multipart upload provides the following advantages:

• You can upload parts in parallel to improve throughput.
• Smaller part size minimizes the impact of restarting a failed upload due to a network error.
• You can upload object parts over time. “Once you initiate a multipart upload there is no expiry; you must explicitly complete or abort the multipart upload.”
• You can upload an object as you are creating it.