Why do you need to replace the backplane? Whether you rebuild the drives before or after replacing the backplane depends on why you are replacing the backplane.  If you are replacing the backplane because you have isolated an issue with it, then rebuild after - get the potentially-faulty hardware out of the picture.  However, if you are just replacing the backplane for fun, or "just in case", then it doesn't really matter, because it's pretty unlikely there is a problem with it. Why are you replacing your disks?  If you are replacing the drives with larger ones in hopes of making your RAID 1 bigger, then you should know that it won't work that way.  The only way to expand an array is by adding disks to it.  If you replace them with larger ones, your array will stay the same size, and the only thing you can do with it at that point is create a second array across the disks, which will show up in the OS as a separate "disk".  Which controller do you have?
I need to replace the backplane and disks of my server. Which steps should i take first: 1. replace the old backplane with the new one. 2. remove old disk in #1 and put in a new one and let it rebuild. 3. remove the old disk in #0 and replace it with a new one and let that rebuild.  Or should i try to rebuild the disks first and then replace the backplane?
replacing disks and backplane