What are these?
An interposer is an electrical interface routing between one socket or connection to another. The purpose of an interposer is to spread a connection to a wider pitch or to reroute a connection to a different connection.
An example of an interposer would be the adapter used to plug a SATA drive into a SAS backplane with redundant ports. While SAS drives have two ports that can be used to connect to redundant paths or storage controllers, SATA drives only have a single port.
Directly, they can only connect to a single controller or path. SATA drives can be connected to nearly all SAS backplanes without adapters, but using an interposer with a port switching logic allows providing path redundancy
Think of Dell PN939 / HP592 SATA Interposer board, Figure 1 below, for usage with CC852 trays / caddies to be used with SATA / SATAu Hard Drives in Dell PowerEdge 2900, 2950, 6900, 6950 Servers and PowerVault MD1000 and MD3000 Storage Arrays, as a practical example.
Figure 1: An example of an interposer board. Dell PN939 / HP592 SATA Interposer board available and in stock at Tekmart Africa.