What server/model is it?
HP ML350 Gen 10
OK, looks like there are many storage options on that machine and many different raid controller options as well.
But, there are no SAS expanders on the backplanes. Backplanes are straight "dumb" wiring so to speak. HPE has an additional SAS expander card that have to be installed if you're using that.
So if you have 16 drive bays I assume 8x2.5" drive bays per "box". You then probably have it paired with the P816i-a controller in which case you have each drive bay directly wired to one SAS port from the controller.
So it makes no performance difference how you install the drives. I would however put the SSDs in one box (one backplane) and the HDDs in the other - just to make it less confusing and less of chance to pull the wrong drive when something fails.
I don't understand why you would use 2.5" HDDs at all though but that is another matter. The server can also be configured with one 8x2.5" and one 4x3.5" bay which would be my preference.
Yes, it is the HP SAS expander card I'm using.
Since the backplanes in that server doesn't do anything, it makes no difference in what bays you put the drives. Not from a performance perspective and not from a redundancy perspective.
From the SAS expander to the backplane to the drive is electrically the same as just having a straight cable. So there is nothing that can fail - unless you physically abuse it.
The backplane does have some electronic components on it though but that is for driving the LEDs in the drive bays.
Awesome thank you everyone!