I wrote above that the volume can be pulled over the extension with the 716. ..... only works with the 7 series, i.e. also the 716. What I don't understand is the statement ' lose 2x50%. This is true if the plates are mirrored on the 716, but not if there are three plates in the extension. There it would be one of three (33%) one of four(25%) or one of five disks(20%). In a 5-disk extension unit I would definitely install more than 2 disks, otherwise the small DX would also suffice. The power failure should not be a problem, because I would never operate a NAS without a UPS. Otherwise, after a power failure, more than one disk would most likely be defective. It happens more often than you think that individual hard disks simply fail. It has happened to me twice in 8 years and the forums are full of questions about it. If one disk fails in a 716 and there is no mirroring, but both disks are in the same volume, the backup must be made and restored, otherwise the data is gone. If there is a mirror, take out the defective one, put in a new one and repair it. It's much easier. As I said, you need a backup anyway.

If I create two volumes, then I can mirror the disks on the syno and create a Raid 5 on the extension, for example, with the fail-safe effect of one disk. This way I lose one disk on the syno and one on the extension. If I make a volume as a stripe, I don't lose any storage space, but I also have no failsafe and the data is gone if I lose a disk in the set. I could also make a volume over all disks and create a Raid 5 or 6 from it and lose one or two disks of storage space, but gain reliability. So it depends primarily on which raid I choose or, if I have two volumes, which raids, and not whether I create one or two volumes. By the way, the fact that all disks can be combined into one volume is only possible with the 7-series. With the 2 series, the extension can only be created as a separate volume. I use my extension as a separate volume with Raid 6 and make my primary backups on it. The secondary backups are made off-site. In other words, please also make backups despite the Raid.