Have you done anything yet, such as:
- unplug and reseat all SATA and power cables?
- run any available manufacturer's disk diagnostic software (from Toshiba)?
- checked for and installed any firmware updates for the drives?
- checked in Disk Management, does Explorer not show the drives but perhaps Disk Management shows disk numbers?
- checked to see if you have Windows power options allowing the drives to turn off?
Hopefully you've done a system backup before trying to do anything with the drives. Last time I had drives disappear in Windows it was because they were in the beginning stages of going bad. In that case, I also noticed that the system would hang for a while during bootup with no obvious cause. I realized that was the problem when I disconnected the drives and the boot problem disappeared simultaneously.
Davemon50