I also just chatted with support and they said the same thing after a webex and checking the array.  The disk is large and will just plainly take a long time.  It went from 21% to 22% during the time we were watching it. I ended up getting commands to check it for myself so I can keep an eye on it.
Hi.  I just replaced my first ever failed hard drive in my Unity 400 yesterday.  The disk which was replaced was a 6TB 3.5inch NL-SAS 12Gbps in a RAID 6 with seven other drives and then a hot spare.  So the whole thing is nine drives with one as a hot spare.  Presently there is 15TB used in the raid.  Anyway, the drive has been showing that it has been resynchronizing for almost 24 hours.  Is this normal?
Unity 400 Disk Resynchronizing