Since you only have USB2 and only Thunderbolt 1 I doubt you find anything that you unit can support that data rate. But then I a a PC person.
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the 320 MB/s seems really high. when i use aja data rate calc i have to select 4k @ 60fps to get close to that. if that's what you are using then you may want two ssd's. one for os/apps/cache and a second ssd for project/media. another option is to use the internal 500gb hdd for os/apps and use an external thunderbolt ssd(s) for cache/projects/media. if you are using HD media around 30fps you may be fine using an external firewire hdd or thunderbolt hdd for projects/media. there are also 2-4+ drive bay thunderbolt enclosures that can hold several combinations of ssd's and/or hdd's, including raid configs. it might even be possible to use a thunderbolt ssd as the boot/os/apps drive instead of tearing apart the imac to install an ssd internally. i think that internal 500gb hdd is going to be slow, so it may only work with lower bitrate HD media. thunderbolt 1 is 10gbit/s so it should be able to handle ssd's and fast hdd's, where as the firewire and usb 2.0 can't.
i don't use mac, so i'm not familiar with your machine and mac os. there are some video's on youtube for upgrading the imac and it looks like it requires alot of work. if you are willing to do that, upgrading to one or two internal ssd's might save some money as external thunderbolt drive enclosures are going to be somewhat expensive. i think i read somewhere only mac ssd's use trim, but owc drives work properly. its something to look into, if you can get any ssd to work properly in the imac the samsung 850 evo's are good drives.
color grading normally requires fast video cards, so whichever gpu is in that imac you may find the performance somewhat slow and faster storage may have limited impact. even with an i7 cpu in that imac the cpu may also limit performance, as those are much older intel cpu's. if you plan on doing more color grading in the future you may need to look at using a better machine. a windows pc is cheaper and often a better option as the mac's are more difficult to upgrade. sometimes its not possible at all, like the mac video cards. if you don't want to use windows but want something affordable and/or powerful enough to do color grading, you could look into building a hackintosh. also if you are focusing on color grading you may eventually want to look into davinci resolve, its free version may be all you need and is much better than premiere and speedgrade.
Thank you so much for your help.
I found the bitrate from VLC, it seemed odd to me too since I shot Full HD at 30 fps. I put it now in Quicktime and it says 185MB/s.
I know that adding an SSD in iMac is quite difficult but on the other hand an external SSD wih thunderbolt is much more expensive. But if I'm going to use it for OS and software then I guess it'd be better to put it inside.
So if I'm correct, you suggest that I use the fastest drive for projects and media rather than run the OS and software?
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sometimes the projects/media drive is the fastest drive and sometimes its the slowest drive. it depends on budget and project needs. by "full hd" i take it you have 1080p media, which should be ok on a medium to fast hdd and firewire. so you could try using the drives you have now and try the cache on the firewire or internal hdd. once you start color grading chances are very good that the video card will bottleneck the system and drive performance won't matter.
if you do decide to get a ssd, a single 500gb+ ssd may be fast enough for os/apps/cache and some media types as well. a single fast ssd may outpace all of your existing drives combined. some of the thunderbolt enclosures start around $100 for a single 2.5"/ssd case and around $150-$200 for one with a ssd included. empty multi-bay drive enclosures jump up to $300-400+. thunderbolt 1 is 10 gbit/s where as that imac internal sata connector is either 6 or 3 gbit/s. so you want to do some internet research on booting from thunderbolt and thunderbolt ssd performance with similar imac's, to see if the cost of an enclosure is worth using vs the trouble of installing a ssd internally.
Ok, so from what you say I understand that it's not necessary to have an SSD which is fine because it means less money for me. I also understand that the most important factor is the GPU which is slow on my system. I just wanted to be able to color correct and not grade my footage. So far my system lags after a while and I thought the SSD could solve that. I'll test using the firewire HDD for the cache.
Generally speaking is it preferable to have the faster drive for media/project or previews/cache/exports?
if you stick with some of the non lumetri color fx, your gpu may handle a little bit of color correction. lumetri is from speedgrade and is used to powerful gpu's. you say the system lags after a while, if that is on returning to the same project that you have already been working on you might want to check memory usage to see if the 12gb of ram is being used up. if it works fine with a new project with this footage and then lags after you add color fx, it might be the gpu slowing it down with the color fx. there are supposedly utilities you can use on the mac to check cpu, ram, and even gpu usage to help identify bottlenecks...
for media, previews, and exports the drives just have to be fast enough for their purpose with the codecs being used. if you have a fast media drive but are constantly rendering previews to a slow drive, it may be counter productive. if the media drive is slow and the preview drive is fast but you don't render previews very often it may also be counter productive. its preferable to have a ssd drive for numerous small files, like the os/apps/cache files, as hdd's do not handle lots of small files well.