My HP Proliant ML110 G7 has a full-size SD slot on the motherboard. What is its use case?
The PDF manual mentions it on page 10: item 17 just to show its placement, but nothing more. In a later revision (Gen 9), it is said the slot is not hot-pluggable.
The motherboard also has a USB slot (item 11).
It's for booting a hypervisor or lightweight operating system like VMware ESXi.
See: What happens when the USB key or SD card I've installed VMware ESXi on fails?
By installing an embedded hypervisor/OS on the SD card, you leave all the front-bay disks available for data storage. This is an useful logical and practical separation.
Some servers have two RAID1-ed SD slots: if a single SD fails, the system continue to run from the mirror, leaving you the time to replace the failed SD.
The most widely installed embedded hypervisor probably is ESXi, with a footprint less than 2 GB.
You can use the SD card for example to outsource the /boot partition. e.g HP ProlianG8 G8 can not boot from HDD when the RAID controller is configured to HBA but you can still boot from the SD-Card.
Also you may by able (not tested) to outsource EFI fat partiotion to the SD-Card to by able to create a software RAID and load kernel / init from there.
FYI: Unfortunately the internal SD Card slot on Supermicro motherboards cannot be used for OS / boot partition and according to Supermicro is reserved for internal debugging use only
Some years ago, the embedded PC motherboard used in a product had a CF card slot on the motherboard. The SD is probably a modern update to that.
For a device that will run embedded firmware, not a desktop or conventional server (e.g. a specialized medical imaging printer) this is a specific feature desired of the board.
As I recall, it was running Embedded XP, an older MS product for that purpose.
