My next attempt (on bsd 10) after searching loader.conf in /boot was to set there
two lines fore ext2fs and ext4fs. So my loader.conf now looks like this

Code:

# * IMPORTANT NOTICE *
# Run 'grub-mkconfig -o /boot/grub/grub.cfg' after making changes to this file
###############################################################################
drm_load="YES"
vboxguest_load="YES"
vboxvideo_load="YES"
crypto_load="YES"
aesni_load="YES"
geom_eli_load="YES"
# Tune ZFS Arc Size - Change to adjust memory used for disk cache
vfs.zfs.arc_max="512M"
zfs_load="YES"
ext2fs_load="YES"
ext4fs_load="YES"
twdm2_enable="YES"

The last line was recommended after reinstalling ext2fs.
By the way, my shell allways sets my last commands to zero after rebooting, how to change that?
The up above notice to run

Code:

'grub-mkconfig -o /boot/grub/grub.cfg' after making changes to this file

did result in nothing, saying that the command

Code:

grub-mkconfig

was not found.
So what to do?

Every time I tried to mount the 2nd hdd, there appears the message, ain't got no rights to do so.
So installing the whole system again, at the point where you can add a disk, I did so, adding even a slice in the format ufs. But again, no rights to mount the volume.
As root in the termninal changing the fstab brings me even more trouble. Rebooting and I am hung in the recovery mode unable to comment the line out in the fstab. I guess this is due to sudo as a file in all that packages. Within the terminal I can modify the file, rebooting, sets me to nothing.
Reading about this, installing in /usr/ports etx2fs and ext4fs
which are already there. So the kernel-bakery was not necessary.
After that I moved on to

Code:

 /boot

looking at

Code:

loader.conf 
loader.conf.pcbsd 
loader.rc

By the way, I set in the first two under

Code:

# Load File-System Support
ext2fs_load="YES"
ext4fs_load="YES"

May I should apply as well to

Code:

# Load Linux compat
linux_lvm="YES"

I really don't know.

After that I had a look at

Code:

 /dev

where the second hdd is listed as

Code:

 ada1

and

Code:

 ada1s1

. So it is there.
After that looking at

Code:

 /etc/rc.conf

and

Code:

 rc.conf.pcbsd

The last one is warning not to write anything into it, 'cause updating the whole thing will overwrite it again. So any changes should be done in /etc/rc.conf

And right here in

Code:

 /etc

I see fstab. But I made this experience before, any changes to this file are leading to nothing. I guess the reason for this is an installed sudo (never mind on debian jessie you will have fakeroot, calif and sudo, so you may read all that complaints of apple users and sudo).
I once added a line to this fstab, and my day was done, hung up at reboot. Even changing file permissions to 775. So reinstall all that system again.
Now looking at gsmartcontrol, my 2nd disk is sound and safe, no bad blocks, it is just not accepted to mount it.

Reading now the freebsd manpage about gpart and another article, there is a hint to add a slice to the 2nd hdd, starting at 1MB, just to have another partition table other than mbr on it. I did so at reinstalling the sys, resulting in nothing again. So my next attempt would be to read the next days gpart and test it with an usb-stick, unless someone here has got another idea. To cut a long story short, is it a good idea to add a slice to the 2nd hdd, beside the mbr that is already there, just to gain access to it? Or how to copy the content and after that format it? And how to gain access to it? That is the crucial point.