Intel NUC only detects one built in HD after my HA installation

I have set up HA on a Intel NUC with two built-in SSD-drives.
The NUC used to run Windows, and was using both drives (as C: and D:)…
.
After my HA installation everything is working just fine, but HA only detects one drive (the system drive itself)…

How can I detect, and use the second drive, for backups etc. ??

You can’t. Not in a supported way anyway.
There is no supported way to connect a second drive directly. You need to consider one of the supported methods.
Mounting drives can VERY much break your system. If the drive mounted brings the backup size bigger than available, for instance, the drive will fill up and the system will crash.

That is why this is not supported and if you do find a way to do it, you will be using an unsupported version of HAOS and you will be on your own when it breaks. It is absolutely not recommended behavior, however it is not impossible to do.
Plus if you really want a drive mounted, install core or another install method where you are in control of the OS.
Clear up storage - Home Assistant.
Common tasks - Operating System - Home Assistant.

Using the supported Samba Add-on is the supported and much safer way to transfer files. You can also use one of the SSH Terminal Add-ons to do this.

The only thing that may work for HAOS in your case is if your system can at bios level create a raid on those 2 sticks, HAOS would see them as one drive and would possibly work. It would have to be all BIOS supported or hardware supported raid, however.

OK, thank you for the information…

It would have been handy to use the built-in spare disk, but according to your recommendations, i will continue to use Samba for my backups…

It’s never a good idea to have backups (only) on the same machine anyway… if machine dies it’s questionable whether you’ll be able to reach such backups.

I did find this later. I may have missed a way to do this…
The data disk has to be larger than the source disk though.