The default when I start installation, just plug in the USB, start and let it run, is GPT and UEFI. If I open the boot menu I can choose BIOS or UEFI. I've tested both, but it makes no difference, except that the rolling text and the menus get larger (lower screen resolution) when running plain BIOS.
However, I just got some new input.
I was out and not far from me there is a shopping centre and they have a small quick-fix computer workshop that was open today. So I asked a guy there and told him that I'm trying to install a BSD system. He didn't know what BSD is, but when I told him that it's a kind of UNIX he got the picture.
And this is interesting. Before I could even ask, ha asked me "is the harddrive disappearing?"
Then he told me that he's no expert in this matter, but he knew that it's a common problem on newer laptops and notebooks when you try to install something else but Windows.
He also asked if I had disengaged Secure Boot and engaged CSM, and I told him I've done that.
The solution he mentioned is that I would need some SATA- or RAID-driver. He agreed that it's a problem with either the partitioning or formatting and told me to contract Asus and ask more precisely.
I didn't really get that thing with SATA-RAID, but then I was thinking that if I would install some driver and then try to put FreeBSD or TrueOS on top of that, it would probably erase that driver. And I don't know if it's possible to go the other way and install a driver on top of the OS and if that could attract the disappeared harddrive again.
As a last resort I've started to think to contact the shop where I bought the computer and try to replace it. Sadly 10" notebooks are hard to get by nowadays. But I saw that they have a HP 11,6" with the same performance for a fair price. But then I have to know that it will work before I try to do the deal. I think they should agree since I would have to ad some money on the replace.
It's this thing, or close to it. Downside is that it's larger and heavier than mine which is very slender http://support.hp.com/us-en/product/HP%20Pavilion%2011-n000%20x360%20PC%20series/6796818/model/6970533/product-info?jumpid=reg_r1002_usen_s-001_title_r0001
@jmaloney Can I use the PC-BSD Bug Tracking System (I already have an account there, but it seems inactive when I just looked) or shall I create a new account on FreeBSD?