this really depends on what your budget is, how much storage space you need, and the type of media that will be on there. with that info others might have a better idea where to point you.
ssd is great if you can afford, samsung 850 pro or the intel 730 series are best. after those two brands the reliability takes a drop off. avoid ocz brand for ssd. hdds the western digital blacks are nice for the price. stay away from seagate, again reliability. you can use a wd green for the backup. depending on the enclosure, you may be able to place the media and backup drives inside one thunderbolt enclosure. some enclosures have raid built in, some let you access all the drives and you would need to use software raid. im a windows user, so im not familiar with the software needed for that on mac.
I am currently editing footage from a 7d dslr so it isnt overly demanding. I just know that in the near future I am upgrading to more advanced formats and these will be more demanding when it comes to the post production process. I am hoping to keep my budget under 600-700 initially but with room to expand down the road. Possibly getting something I can add more drives to down the road or upgrade the ones I get off the bat? Like i said I am very new to this aspect of post production but I just learned how big of a part the appropriate drive set up can play when it comes to editing! so any info or tips are greatly appreciated. thanks!
if you are doing mp4 dslr footage, then fast hdd's wont be an issue. mp4 is so compressed, it will actually take more cpu and gpu power to decompress them while editing. if you are doing a raw or other high bitrate off the dslr then you will want super fast. this continues with the "advanced formats" you expect to be using in the future. the higher the bitrate = larger the file for same duration = more demand it will place on the hdd to read all that data. some of these formats may still be ok with hdds, some may need ssd... hdd vs ssd all depends on how much how much space you need, vs cost.
for your price, you might have to get what you can find at that price range for a thunderbolt enclosure. there are a few 4 and 5 bay enclosures under $500, but i don't know any by brand to tell you any better... at close to $500 for the enclosure alone, that leaves room for just (1) 2tb wd black and (1) 3tb wd green. a 4 or 5 dock enclosure leaves room for adding a raid and adding to backup. if you think your projects will be small and dont need alot of space, you can look into an ssd, and add another in the future in raid to expand its size when u get to the "advanced formats". again, im not sure what software you would need for raid, if you can do that with mac or need to purchase extra software.
maybe someone here can recommend a good 4 to 5 bay thunderbolt enclosure for under $500?