Network speed limited to 50% of the bandwidth?

"According to your description, it can be either: router, hard disk, external disk, network card.

You could try to connect directly to your external disk, to find out if the bottleneck is the hard disk or the external disk. If it isn't, then you could connect directly thru ethernet to another computer to find out if it's the network card.

EDIT

If the other computer is connected by wifi to the router, 5.5 MB is very respectable speed, close to the theoretical maximum (which is 6.75MB/s)."