I was wondering what about my Mid-2009 Macbook makes optical hard drive enclosures like this one incompatible with the enclosure. And if there is another way that could get a second drive in there.
according to this site, it is compatible but there are some limitations:
2008/2009 MacBook/MacBook Pro 13", 15", and 17" models. (Model IDs: MacBookPro5,1; 5,2; 5,3; 5,4; 5,5 and MacBook5,1; 6,1; 7,1) While a 6G SSD does function, it will only do so at SATA Revision 1.0 (1.5Gb/s) speeds rather that the SATA Revision 2.0 (3.0Gb/s) speed the machine can deliver. Should owners of these machines desire another SSD option, the Mercury Electra™ 3G SSD does run at the full SATA Revision 2.0 (3Gb/s) specification.
What this means is that your macbook does not have a connection fast enough for most ssd's and you won't be able to utilize the full potential of such a drive. You can always contact the guys at owc and ask for a recommendation for your mac.