Resync: Reconstructing a disk from the others. This always is done when a disk is replaced.
Restripe: Reorganize the RAID structures across all the disks. This is needed whenever the volume is expanded.
I suspect you are using flexraid/RAID-5 on your NAS. If that's correct, you had a single RAID group of 4x2TB when you started (6 TB volume), because the disks were of unequal size. The first disk replacement would simply reconstruct the contents from the other three - a re-sync. The second would also expand the volume (because all the disks are now of equal size). That involves creating a second 4x1 TB RAID group (which needs to be striped). That group is then concatenated with the main group, and the file system expanded.
If you are using XRAID, then you'd already have two RAID groups. Both disk insertions would include both a resync of the 4x2TB RAID group, and an expansion/restripe of the 1 TB RAID group. Before the first disk insertion, the second group would be 2x1TB RAID-1. That would be converted to 3x1TB RAID-5. The second insertion would expand/restripe that RAID group again to 4x1TB RAID-5. If this is your situation, then it's not clear why the log calls the first expansion a resync and the second one a restripe.
