PC only works with 2 hard drives

[casipy]

Hello,

I've a PC with 2 hard drives (seagate st3500620as) and only works when both are connected I want to change one for a bigger one but didn't work only one of then.

 

[scorpinock2]

Are you in a RAID configuration? If so that's the cause. If not then make sure your not replacing your boot disk.