Hello again JorgeB,
Just to report as you suggested i replaced Disk 3, took about 8 hours to rebuild a 4TB drive and my problem is solved.
In my defence this wasn't the first hard drive that malfunctioned. The first drive i changed, i had a notification on my "dashboard" and a "red thumb" warning.
On this drive i had a "green thumb" and no notifications warnings, just slow, very slow speed when performing the "parity check", this completly throw me off.
Once again thanks for the tips.
And now a print of my normal speed 170MB/sec , not 46 days 513kpps