Routers that allow you to attach a USB drive for file sharing have typically measured around 5 MB/s for read
. This is around 40 Mbps, which should
be fast enough for HD streaming. But I haven't tested this, so no guarantees.
Most NASes run open source based OSes and use EXT, XFS or other similar drive formats. There are programs that allow EXT drives to be read on a Windows machine.
If the USB drive is FAT formatted, you probably won't notice a performance hit, especially since the performance is so low anyway. NTFS might not be allowed for read/write, you need to check the router's specs.