Over ssh, I would use rsync. This brings everything over uncompressed, but it will only bring over changed files, which is likely what you want as well. A 600GB transfer will probably have a few connection drops. This recovers automatically.
I've used variants of this script over time. I forgot where it came from, so if the author is found, I would like to give credit.
### Runs rsync, retrying on errors up to a maximum number of tries.
### Simply edit the rsync line in the script to whatever parameters you need.
# Trap interrupts and exit instead of continuing the loop
trap "echo Exited!; exit;" SIGINT SIGTERM
# Set the initial return value to failure false
while [ $? -ne 0 -a $i -lt $MAX_RETRIES ]
rsync -avz --progress --partial -e "ssh -i /home/youngian/my_ssh_key" /mnt/storage/duplicity_backups firstname.lastname@example.org:.
if [ $i -eq $MAX_RETRIES ]
echo "Hit maximum number of retries, giving up."