Yes, that's the one. I've haven't tested it but I think if you have this you'll be fine even on the first big transfer (I think). But I want to rephrase my previous statement a little. In my previous statements I've been saying this is an issue only on the first big transfer, but that's not correct. Once again to be clear this issue happens if your split level is not set to "Automatically split any directory as require?" and you're trying to rsync a large batch of files. It don't even have to be that large of a transfer to trigger this. What happens when you do a rsync transfer is that rsync will create ALL the folders on the first available disk according to allocation method BEFORE transferring any files.
Split level is about trying to keep files together so they don't end up on different disks, so if you have the wrong split level unraid will try to force some files to a disk which is already full, trying to keep the files together according to set split level. Since this is just a backup I wouldn't worry about where files end up anyway.
So if you take those two paragraphs and put them together you'll see that this issue can happen on even a small transfer. Lets says you have transferred all your files and in 2 months time you have 500 new folders with pictures in them, roughly 5 gig in total. Which you want to back up. The next disk in line to get new files based on allocation method is disk 3, which only has 2 GB available space left. Your split level is set wrong and because of that and this rsync issue all 500 folders will be created on disk 3, even if some of the files in those folders won't make it to disk 3 because it's completely full by the time rsync gets to them and it won't choose another disk either because split level goes before everything, so the transfer will just fail with out of space errors.
So for this to be set it and forget it you will have to choose the first split level like mentioned, you will have to set a min free limit so that unraid will choose another disk if that disk has less space then the limit. Since this is just a backup I think I would set the allocation method to fill up. If you're not concerned about evenly filling the disks that is. I try to keep at least 30 GB of available space left on all my disks.