What should I use for a boot drives?

You can use either.

But it might be best to use something that is easy to handle and easy to replace. And easy to have at least one spare for.

In your place I'd go for USB thumbdrive. You can easily get another and clone them using any PC.

But if you have a PC with a SD card reader, then that is true for SD card as well.

I'd avoid using a used SSD. It is possible that would work best and for a long time, but it may also fail at any time and it is not as easy to have a backup SSD.

If you really need to backup the OS drive every day you are doing something seriously wrong and your thoughts about using flash memory for the OS can be scrapped. Then you need a SSD.

On the root filesystem I ONLY have apps and configuration. No, or at least very, very little, data that change often, This means that I only need to backup the root filesystem when I'm doing major upgrades that perhaps also involves configuration changes. Perhaps once a month or half year? Then it is easy to shut down, remove the USB drive and clone it. That is exactly what I do, but with SD cards.

I have a current cloned copy of all cards, ready to plug in, as well as images of all the cards on my laptop, for making more clones. (And backups of the laptop on a NAS.)

The data drive may need much more frequent backups. Even daily or more frequent.

Automatic backups are a nice recipe for catastrophic failure. I know first hand!

Better might be quick and easy manual methods that automatically verify or even guarantee that the backup is good. For instance clone the USB drive after reconfiguring and/or major upgrades. And then switch the original and the cloned copy.

This is only manageable if the need to clone is not too often. And that it can be accomplished quick and easy. This can be achieved by very strictly making sure that you don't have anything changing on the root filesystem. Which also is necessary if you use SD card or USB thumb drive.

That said, even if you loose the root filesystem, it should be easy to reinstall and configure from scratch. Taking a few screen dumps may help.

Zitat von cabrio_leo

Another reason to create regular backups of your data

... and a very good reason not to use RAID.

Besides, with RAID you may risk loosing all data on the whole array when only one drive in the array goes bad. So even if you use RAID, take backups!

Zitat von silk186

I don't have experience working with any type of RAID. I know if a hardware control dies, recovery of the array can be problematic.

For these reasons, don't do RAID (of any kind) without full data backup. Instead, use the disks that would make up an array to create the 2nd copy (of your data). You'll be ahead of the curve and it's a bit more cost effective.

Zitat von silk186

I assumed that if my thumb drive failed it could also be difficult to recover without a recent back-up.

If you think about it, basically, this is not true.
The only time you need to update your OS (thumb-drive) backup is if you make changes to your configuration of the NAS. What does that mean? If you do generic security and other Linux updates to the NAS, (php, python, grub updates, etc.) there's no need to update your clone. But, if you add a new shared folder, setup a new service, add a new plugin, a new Docker, etc., these actions would fall into "your configuration". This is when you'd need to re-clone your spare and, after you settle on the configuration you want, this will be fairly rare.
But, even if you didn't update when you should have, all that would be necessary would be to make the missed configuration changes to the older clone. (The data, in a new share for example, would still be there.) This would still be far easier than a full OS build and reconfiguring, from scratch.
(BTW: When to update a clone is in the guide PDF.)

I suspect that it is VERY easy to implement. But I also suspect that if you need to ask you shouldn't do it.

Note that if you have N drives, in a JBOD arrangement, you have N times higher risk of loosing ALL data in case of a random drive failure. So you then REALLY need to have good and recent backups.

Zitat von silk186

I'm thinking about JBOD, is this simple to implement?

I hate to say this but, "it depends".
In my opinion, JBOD at the physical level, is the way to go. These days, even if you go with RAID at some point, it's still best to let software assemble the array from JBOD disks. Otherwise, with hardware RAID, your disks are permanently married to the controller or a specific family of controller.) If it's a software array, you can move them (the disks) to a different hardware platform.

On implementation, really, "it depends" on your controller. It appears that you bought a commercial server which, if you ignore the power they use , are a good deal - high quality, high end, server grade components throughout. (I have an Intel SC5650HCBRP that's similar.) The Intel came with an Adaptec controller (a PCI card) that, while it would do JBOD, it insisted on "initializing" every drive attached to it. Along with not passing SMART stat's transparently, this made using the stock controller a No-Go.
That said, I suspect that your embedded RAID 0, 1, 10 controller may not have those issues. (I'm fairly sure it doesn't.) Set the controller to JBOD and see what happens. Thereafter, if you see the drive in OMV's GUI console, without having to do anything in the controllers BIOS to add a drive, you should be good to go.

Beyond that, commercial servers are usually very well documented. See what you can find in the way of documents on HP's site. If you find your drive controller options are limited, take a read through of this -> thread. Flashing a RAID controller to IT mode is an intermediate level task, but the "bang for the buck" is excellent and you'd have a server grade controller with 8 ports at very low cost.

Zitat von silk186

It would be much easier to manage a single volume.

If you want a single "mount point" that is similar to RAID, one of the safest routes to get it is with UnionFS (provides the common mount point) + SNAPRAID (just one disk provides for disk replacement, parity protection, file/folder restoration, and bitrot protection. There's no other way, that I'm aware of, to get all of those protections from a single additional disk. And nearly of the housekeeping functions, scrub's, etc., can be automated in Scheduled Tasks.

BTW: If you're storing a lot of video files (very large) change the default storage policy in UnionFS from "Existing path, most free space" to "Most free space". You'll get even file distribution, among all drives, that is more suited to the purpose.