Adding a NAS to my network

NAS setup

Without knowing your setup better. I would suggest that you connect the NAS to the Airport Extreme via wired ethernet. That is the way I have my Airport Extreme connected and I use Time machine and it works great.
bigbearf

 

An Airport Express's USB port is for printer sharing, not a NAS. If the hard drive you want to use for a NAS has an Ethernet port, then you would be ok using the Airport Express as the entry point into your network.

However, as previously stated, you would be better off colocating the NAS hard drive with your Airport Extreme and plugging it in there - either to the USB or to the Ethernet port. That greatly simplifies your networking situation. Depending on what version of Airport Extreme you have, you probably have Gig-Ethernet which might give you some more speed. The Airport Express only has "Fast Ethernet" (100 megabits).

I have two Airport Expresses that I use for Airtunes and a separate base router for Internet and several hard-wired connections.

 

Does your Airport Extreme have the latest firmware? I ask because I put a USB drive to my Time Capsule and can mount it on each drive, art the same time. You can even put a power USB2 4 port hub on that port and put drives or printers on it. I haven't used that drive because I have a local backup on my Mac Pro and for my Mac Book Pro. The external on my Time Capsule is used as a media share to all my LAN devices.

Now I gave my friend an Extreme for his Birthday and he put a USB drive on it and use Time Machine on that disk. He was able to do this after apple released a firmware update around a year ago to do this. It is just not supported officially though but it will work for Time Machine. Just backup locally connected to the Extreme for the first Time Machine backup. Then disconnect, the initial backup will be huge and will take a while. After that yopu can backup through the air because those incremental updates will go fast after that, through the air with no problem.