mac mini drives are terrible. i wore out like 5 of them
if everything Deltamac doesn't fix it then you have to replace it
in the old days you could open up the drive and try using a swab and alcohol on the laser , but that not really doable i tried it
mac mini and macbook pro share the same drives, so if all else fails you can use either one and replace it
mac mini would require you to unscrew macbook pro mounting parts. you can get optical drive pulls for these computers cheap on eBay
apple sells an external drive with 1 usb connector , but since your mac is an older 2009 series, i wouldn't count on this one working. pre 2010 macs are limited on usb power.
apple's external superdrive requires 1100 ma and i don't think your 2009 is capable of putting that out. but i could be mistaken, i know for sure my mac mini 1,1 2006 couldn't put out anything more then standard usb current