Openmediavalt 4 not seeing MegaRAID SAS 2108

Not 100% sure about this but I think raid controllers will expose the raid only. As I understand those controller cards you have to change the fw to expose each disk individually. IT mode is called.
Someone here might confirm later. IMO software raid is better is more portable.

I agree with @subzero79 . If you really want to run RAID, software RAID is better. With software RAID, if you decide to transfer your drives to another host, they'll be recognized as a RAID array on the new host. Software RAID is much more flexible, when compared to some hardware controllers.

Some cards will operate in "IT mode" which allows transparent drive pass through. According to what I found on the MegaRAID 2108, it has a pass through mode. If set for pass through mode, OMV4 will see the drives and be able create a software RAID array and format it. Otherwise, if the card is used to set up an array, the array will appear to OMV as one large disk, which can then be formatted.

This -> manual for the MegaRAID 2108 may (or may not) cover the BIOS version on your card. You'll need to access the BIOS of your card to set it up for "pass through" or for RAID operation. On the other hand, if you want to use it as a hardware RAID controller, setting up the array is usually done in the card's BIOS.

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In any case, the tech info you need should be available in a Google search.

If you really, really need RAID then hardware RAID is much better. Typically HW RAID allows the filesystem to be online even if one or more HDDs fail. And if the hardware supports hot swap you can replace the drive without even turning off the computer. Also hardware RAID typically is much faster.

Naturally, for hardware RAID you also should invest in staff that monitor the hardware RAID 24/7 and a couple of spare HDDs and one or two spare RAID cards as well. And train diligently in fixing drive and RAID card failures.

If you use RAID for other reasons than having a very high uptime/availability and high performance, then software RAID is fine. But in that case RAID is typically used instead of a robust backup system. In other words the reasons for using software RAID are often dubious.

Any form of RAID means that you do a trade-off where you accept more frequent hardware failures in return for less severe hardware failures. More drives will fail, but you can recover without having to restore a backup.