Which hard drive is master and which is slave?

A computer with multiple hard drives has one natively designated as the "main" hard drive. The computer will naturally boot up from this "main" hard drive. I believe this "main" hard drive is called the master.

When tearing down the A1347 Server model, which hard drive is removed first, the master or slave?

Asked another way, which hard drive is closer to the Apple logo, the master or slave?

I would like to replace the master/main hard drive with an SSD hard drive while insuring default boot is correct.

Thanks.