So you still have some errors with your ata ports and not only during the boot.
Currently you have errors with all used ata port (ata1;ata2;ata3;ata4;ata5).
From some Internet searches, this can go from :
- a material issue
- a speed link not supported
- the kernel setup.
- sata cable (bad plugging or deteriorated cable)
At least, I see that some times your disks are link up to 3Gb/s and other times with 1,5Gb/s.
The dmesg errors means that several times your disk are plugged/plugged
do you open your nas ?
and see if these errors still occur later when you use the nas