so I took a look here:
Base Hardware N40L
and a few challenges here from what I read the N40L uses low profile/half-height card slots. anything you do exapnder wise - if it is a card slot expander will need to be half height. I'm having a hard time finding internal pictures of the N40L so I don't know how much room is inside to mount an exapnder and the attendant cables. By the name microserver I'm going to guess not much room inside.
if you install a 5.25 bay device that supports 8 2.5" drives *and* you want to use the 4 3.5" you'll definitely need an expander *or* a second card in your unraid box or a card in your unraid box that supports 16 SAS/SATA channels (like a 9x00-16i series). Because your N40L will have 12 devices in it. If you were to just install a 5.25 bay that supports say 4 2.5" drives (Note you can use up to 15mm drives now with the right bay device) your H310 will work though it may not be the best choice.
if you want this to look clean with the H310 its going to be something like this:
and be forewarned that with all these cables and connector conversions you may exceed the maximum SATA cable length and it won't work.
install <H310 in unraid> ->
<2x SHORT SFF-8087 to SFF-8087 cable-> ->
<SFF-8087 to SFF-8088 dual port bracket>->
<.5m SFF-8088 to SFF-8088cable> ->
<low profile SFF-8087 to SFF-8088 dual port bracket>->
<2x SHORT SFF-8087 to 4xSATA cable> ->
4 connectors to your new 5.25 bay with 4 2.5 slots
4 connectors to your existing drive bays.
No OS on your N40L, it acts as a power switch . You may be able to remove the ATX power cable and use a plug and if there is an on/off switch on the power supply not even power up the mainboard.
Another option is this, its kludgy but should work. A big downside to this approach though is that if you need to move the unraid server or the N40L you'll have to open the case one of them to disconnect cables.
install <H310 in unraid> ->
<2x LONG SFF-8087 to 4x SATA cable>
open up a bracket hole in your N40L and route the cables into your N40L
4 connectors to your new 5.25 bay with 4 2.5 slots
4 connectors to your existing drive bays.
No OS on your N40L, it acts as a power switch . You may be able to remove the ATX power cable and use a plug and if there is an on/off switch on the power supply not even power up the mainboard.
There are variations and a few different ways to cut this as well so this is only two ideas off the top of my head. happy to walk through others if you'd like.
One thing to be careful of is that you could end up in a money pit trying to make this work. it won't be big money but still likely money wasted.
Before purchasing more stuff you might answer, do you think you only need to add 4 drives for today? do you think you'll need another 4 in say 12 months?
Is sound an issue with the solution? Ie your unraid server and the N40L are quiet and that's important?
solving a problem today that comes back in 6,12, or 18 months may indicate a longer thought process today?
Just throwing that out there.