How to use airport extreme for storage only

By backups are you referring to Time Machine backups or as a backup file repository?

For the former, the AirPort Extreme would need to be an 802.11ac (tower) model to which you can connect an external USB hard drive. The AirPort Express cannot be used for this purpose.

For the latter, again you would just need to use an external USB hard drive that you would share out using the AirPort Utility to configure it as such. Again, the AirPort Express cannot be used for this purpose either.

Just to add to Tesserax comment.. as a backup destination.. a USB drive plugged into an airport extreme plugged into your network.. is going to be slow.. and very unreliable. IMHO don't!!

If you already have a drive and want to give it a go for sure.. but don't spend money on something that is really not up to standard.. and since Sierra.. and particularly High Sierra has proven really problematic.

What do you mean by the last bit.. life is over.

How to use AirPort Extreme now that its life is over

As per Rudegar comment.. are we talking about a Time Capsule or Extreme..

I will assume a TC..

What model is it?

If it is the current AC model and you are no longer using it as a router and Time Machine backup.. explain exactly what you want to use it for.

TC are rather poor as a NAS.. it is neither a real NAS.. nor a normal router.. and it has big limitations.. some of which should be taken into account if you plan using it truly as a file server.

I put some of the warnings into another recent thread.

Airport Time Capsule as a virtual drive

if you mean how to use the time capsules hd as storage rather than time machine

Setting up the 2TB Time Capsule as a NAS. Help needed.

more

https://www.google.dk/search?q=use+time+capsule+as+nas&ie=&oe=