I'm planning to order the 5k iMac and I have a question about the SSD. If I add the 512GB SSD from apple I'll need to pay extra $270 for the drive but I can add a Crucial or Samsung from Amazon with a USB3 enclosure and 512GB for just $200-$220 and that twill bring total capacity of internal 256GB SSD + 512 GB SSD = 768GB total. My question is if anybody know how slow will be that 512GB External SSD with the USB3 enclosure, I do not do any heavy work on the computer, I'm a web developer so I code, design, mail, family videos and photos, etc. Is the external SSD a better deal or it will crash in speed? It should be much better than a regular external HDD right?