there no thing as forever in storage. cd and dvd have a 15-30 shelf life if you keep them dry and in a dark place..google cd rot.
hard drives as long as there not moving they could last years in storage..but most people use them as daily back up. the drive could last 3 months or years. I tell people if you have photos or work you dont want to lose use more then one back up. i tell them use a usb stick then put the stick away where kids cant crush it. I also tell them to use one of those free cloud base or free online backups. you dont need to back up your os but it nice to have an off site place to store stuff if there a fire or flood and your pc and back up disk or hard drive is damaged you have another place to get your back ups from. the easy back up is going to be a hard drive and some back up software...if you need all your program and data then people use ghost or drive clone. if it more then one pc..say a pc..phone..ipad..then getting a cheap network storage and plugging into your router/wifi would be for the whole house.
Nothing will hold forever. Back it up on DVD and HDD. DVDs should last at least 10-15 years; HDD should also last as long, provided it doesn't fail and you keep it away from magnetic sources. I wouldn't go with SSD yet because it's too new.