I have just discovered that my old Freecom Media Center 35 can be used as a Network Attached Storage (NAS)!
I bought this a couple of years ago and I used it a standard external usb hard disk and also connected to a network to play media on a computer with a Windows Share.
But now (I don't know how long it has this functionality, the manual doesn't mention any of this) I found (while searching for a firmware upgrade) out it can also be used as NAS. All you need is the NDAS Software.
To get it working I had to upgrade the firmware to version 1.4.9 first. Then configure the media center with DHCP and NDAS on. Register the NDAS device ID and Write Key on the NDAS Device Management that I had just installed and that's it! I now have a new drive mapped.
You can even copy files while it is reproducing a video, but I don't recomend: it doesn't have enough resources to play the video smoothly while transfering files...