The FDX EMI/RFI Filter

The cylindrical silver component close to the FDX case fan is an EMI/RFI filter. It is intended to protect the FDX from the detrimental effects of EMI/RFI present on the AC power line, as well as ensuring that interference generated by the FDX itself is not propagated to other devices connected to the power line.

I expect that as well as the technical requirement of protecting the FDX from AC line interference and the effect of the AC powered case fan, the filter would originally have been installed to meet FCC / IEC requirements.

As an aside, though it probably makes little practical difference in this case, operation of the filter can be compromised to some extent by poor wiring practices. There should be maximum separation between the power input lines and the output wiring from the filter. The photo above shows that Memotech didn't take much care in routing the wires to/from this filter - other examples I have seen are even worse with the input and output wires running in parallel across the filter

It is also worth noting that the transformer adjacent to the fan is not typical - in this example it is because Memotech had used a 115VAC fan in a 240VAC powered FDX.

This is a typical circuit diagram for one type of EMI/RFI filter.     Diagram from REO Power Solutions website
Is the filter required today? - I think that it depends on the power supply. If the original FDX PSU is installed, I would retain the filter to protect the FDX electronics. However, as I was replacing the FDX PSU with a modern PC supply, then I don't think that the filter is required, particularly as I was also replacing the AC powered fan with a low voltage DC one. Modern PC PSUs have good EMI/RFI input circuits that should cover any requirements for EMI/RFI protection for the FDX.
The input filter stage from a typical ATX PSU   You can read the details of how it works here       Diagram from pavouk.orh website