Hey guys this deal is back on ! very cheap flash memory upgrade for a macbook when combined with this
http://www.ebay.com.au/itm/Micro-SD-Card-Adapter-for-Macbook…
Well this is what I had in mind should be fine as a secondary flash drive and this adapter will not protrude out the side of your mac.
Assuming it is a macbook pro w/CD drive. You do have an alternative choice you can do which i believe is a better solution
grab yourself a Caddy or something similar and buy an SSD slim enough to fit in. Grab one from any PC store, or wait for an online sale.
P.s Yes it will replace the CD drive