Could also be failing HDs, they usually vanish like that...
Make backups just in case.
To have two hard drives disappear within minutes of each other, especially when I was just using them was very strange indeed and provided a brief, but scary moment. Two hard drives gone, a 4TB and 2TB, would be a lot of lost data. I am a firm believer in backups and do have them, but it would still be a PITA to replace everything.
After a moment of panic, I checked under the hood and switched cables from working drives. When I rebooted a couple times, the drives were back on line so it's something other than defective drives. My system is getting pretty old and it's well used, so a SATA channel or mobo failure would not be totally unexpected. Some kind of software glitch also a possibility.
Anyway, I'm thankful that everything is OK(for now at least). I'm glad I have backups and I'm reminded once again how much data you can lose at one time with large capacity hard drives.