Greetings,
Based on the description, it doesn't sound like you have missed anything. You may however want to consider installing the Intel specific RAID driver for your controller (if available) While windows 8 and 10 support the Z97 set in AHCI or RAID (controller mode) natively, you might get better performance using the IRST driver.
A while back, it was always recommended to use identical disks when creating a RAID array. This is no longer true and hasn't been the case for a long time. RAID controllers are extremely robust now and are capable of managing different drives in an array as long as they share similar characteristics. This should be true of the Samsung and SanDisk you have chosen. Of course, the chance exists that one or the other is not a "good match" for the set, but unlikely IMO.
Have you confirmed the health of each individually? Once again, SSD's are pretty reliable. They basically work until they don't. If you are using any Gigabyte App Center programs, or have Ultra Fast Boot enabled in BIOS, I'd change to Normal or Fast only and disable or uninstall any specific (or performance) related apps while you troubleshoot your issues. We assume your cables are in good condition and correct for the install.
May also help everyone here to know what BIOS rev you are running. Memory, CPU and if you are overclocking or not. And/or other troubleshooting steps you have tried.
