thanks for your reply - it works now. I was a little bit hesistant with loading default BIOS settings because I didn't know whether the PC manufactorer made some special settings or not. But I tested it and since it worked with default settings, I wanted to know witch option made the difference. And so I found the "UEFI Boot" option to be the reason (must be checked).
But the point is that I run a debian (jessie) with grub as bootloader (standard version, non-EFI) and both disks are smaller than 2 TB (SSD: 120 GB, HD: 1 TB). Because of this it never came in my mind to activate "UEFI Boot". It still works without the "UEFI Boot" option using the F10-boot-menu or by clicking the disk in Visual BIOS. Even more direct booting works without this option if grub is in the MBR of my hard disk. So it still seems to me, that the BIOS treats the mSATA-SSD and the hard disk not in the same way.
However, since I got my solution I'll mark the question as answered and thank you for your help.