Securely replacing 8 drives on 2 Pro Nas ?

Thank you StephenB for your recommandations.
My only concern is i would prefer to use the new WD red disks on my Pro 6 for performance and cooling reasons because Pro 6 is always running (Pioneer wil run only time to time for backups).

StephenB wrote:
-The resync process on the Pro-6 will be slow .... -If the Pioneer is already only used for backup, then I would do a factory reset on the pioneer, and then migrate the data from the Pro with Frontview backup. The Hitachi's are being pulled over time, so it is quite likely that they won't really be in sync.

OK i understand the "one by one" resync process is dangerous, very long, and above all i would risk to be unable to install the old Hichachi disks on the Pioneer because they will be out of sync. Correct ?

StephenB wrote:
Assuming the Hitachi's are running ok

Pro 6 and Hitachi's drives are actually running perfectly !

StephenB wrote:
- I would probably do a clean install of 4 unformatted WD Red drives in the pioneer, and leave the Pro alone.

OK , so i only have to plug the 4 unformated WD red on the pioneer, boot and factory reset ?

StephenB wrote:
Then create the mirror with frontview backup.

1) Is using frontview backup OK to get a perfect mirroring from the Pro6 to the pioneer including database, addons-config ..?
2) Is it possible to follow your process and then to swap the 4 disks (on same slot location): WD on the Pro and Hitachi's on the Pioneer ?
Regards