Adaptec Raid card 71605 - I have 2 drives that randomly are marked inaccessible

I have an array of 8 wd 10tb drives in my DAS connected to the adaptec card in raid 6. The same 2 drives keep dropping randomly and are marked inaccessible. Sometimes 1 or both. No rhyme or reason. Array rebuilds each time. I double checked the wires which seem secure. Do I have 2 bad drives? I suffered a series of brownouts and surges (its on a ups) that caused the initial problems. Thanks for the input.