Hi SA A,
when the synchronization was at 4%. 15 hours later all the files are copied on the raid but the synchronization didn't moved at all and is still on 4%
Most likely there's a problem with one of the drives. Check the events, you may see command timeouts or repeated bad blocks on one of the drives.
FYI, I worked with a customer last week who had bought supposedly new drives, when we check the SMART attribute 9 (power on hours) most of them had from 95,000 to 98,000 hours. These drives has been used for 11 years. He decided to return those drives and I hope he has better luck with getting new replacements. The point here is to check the SMART for your drives.