This applies to the 3u MDS, a SuperMicro SuperServer 6036ST-6LR, SC937ETS-R1200NDBP, uses the X8DTS-F mother boards.
There are two removable CPU sub chassie modules in one 3u enclosure sharing a common disk & power backplane.
On each CPU module there are 6 sets of cooling fan assemblies, each fan assembly is made of 2 fans mounted back to back with one 4 pin connector for power, monitor and control.
In the front of the 3u, there are Over Heat / Fan indicators on the left and right side, it flashes red if there are any fan problems.
The fan assemblies are arranged in a upper and lower tiers, three fan assemblies are at the lower tier and three are at the upper tier.
The lower tier fans are connected to the mother board, the upper tier fans are connected to a small adapter PC board which then plugs into the backplane when the CPU module is inserted.
The lower tier fans are monitored by the mother board and the status of them can be viewed from the BIOS. The front Over Heat / Fan indicator will NOT reflect conditions of the lower tier fans. If any of the lower tier fans fails, the front LED will not flash.
The upper tier fans are monitored by the backplane, if a fan fails the Over Heat / Fan fail LEDs will blink (on BOTH left and right side! The manual says only the side that fails will blink, which is wrong). The upper tier fans are NOT monitored by the BIOS.
Failures of the upper tier fan will be difficult to locate, you have to look at them to identify which has failed, the fans are difficult to view if the unit is fully assembled and buttoned up.