Syncing After Replacing Failed Drive

[steamship]

A bit of history before the actual question. I have a Buffalo LS441DCF1 with 4 x 4TB drives, which I had set up as RAID0 (to get the max capacity), but did have the sense to have external drives for backups. One of the disks failed and I got the new one today. Deciding to err on the side of caution this time, after booting it up and formatting the new drive, I deleted the failed RAID0 array and created a new RAID5 array. Once it was created, it started a 'syncing' process which it's showing as taking just over 40 hours to complete. Checking the 400 model user guide, it states "The resyncing process takes about 1 hour per terabyte of space in the array". With that in mind, even in RAID0 it should only take 16 hours.

The question - Why is it taking so long to perform the sync process when there is no data on the drives ('Unit Capacity' is showing as 11131.1GB and 'Amount Used' is 0GB)?

[steamship]

Just to update this. Total time to 'sync' 4x4TB with nothing on them took 34 hours! Over twice the time the manual would indicate. Now to restore the files onto them again.