Time Machine and Hard drive

for some reason I am having problems with Time Machine accessing a hard drive i have plugged into the USB port of my airport extreme. I can access the information on the hard drive but for some reason time machine says the disk isn't available. I had this problem before and I reformatted my external, it worked for about a year and now its screwing up again. I don't want to lose all the information on there so is there something else I can try?

I have tried to use disk utilities but nothing fixed my issue.

Use Finder and it should show up once you click on Airport Extreme then click on it then
it will show up on your desktop then open Time Machine and select the drive or at least
this has worked for me.

well actually I have tried it... I can mount it and look at its contents but for some reason time machine will not use it to back up my system saying the disk is unavaliable

In Time Machine was it in your drop down list and did you select it after using Finder, sorry
left that part out.

I go into time machine I can select the hard drive to save onto. I enter my password and i connect. As soon as I go to "backup now" it says the back up disk is not available. I can access my back ups through Time Machine but I have to pick the option to use previous disk. I can access them if i go through finder. Sounds kinda weird to me

Have you tried unmounting the hard drive then plugging it back in?

I have unplugged the power from the drive gave it a minute and then plugged it back in but it didnt change anything.

What I meant was the USB connection eject then plug back in hot.