I've never had the "luxury" of a second internal drive in any of the computers I've used for editing video. From a single core emachines with a 1.6GB drive editing Digital8, to a single core hyperthreading HP with 512MB RAM to a dual core HP with 2GB RAM editing HDV (and with that one I started capturing to and rendering on external USB drives some), to my current Q6600 and Core i7 Dells editing AVCHD natively, I've encountered no problems other than the Q6600 processor being a bit slow.

if you just look at plain number, no hardisk , even in RAID, is able to support video editing.
fortunately , nobody use "uncompressed full bandwith" to edit video.
So the real numbers are the one used to record compressed video. (between 4 to 100 Mbytes/sec, but usually well under 50)
And since there is no tape , disk or memory that can be run fastest than a good harddisk, you can expect that Disk on Firewire or USB could fit the task.
Now, while any car can fit the task of bringing you somewhere, for sure, some will do it with better safety, speed , comfort or ease. same with computer equipment.
Depends your budget and expectations. Just make sure both will meet.

A simple rule that I used for years is do not read and write to the same disc at the same time, one stream per disc. This rules out using the OS drive for any video storage or temp files and also means having at least three discs, preferably four or more. This also means that external USB is just fine under these circumstances. IF you have more than one external make sure they are not on the same controller which for some motherboards may not be possible. I have used external USB2 for DV, HDV and AVCHD with no problems. Though now have no need to do so with my present system.
My current configuration is 250G OS, 250G temp, 2x750 RAID0 video storage, 1TB drive for finished projects. Backup to DLT03 tape drive. Also have several bare 1T drives that I use with eSATA when needed using Thermaltake disc dock.

Ron Evans

I use an external (well, several) FW400 external discs for capturing and editing Apple ProRes compressed video sourced at HDV720P60 and I'm writing and pulling 1GB per minute (ProRes 720P60) with no problems. Currently I'm on an iMac until Snow Leopard comes out so I don't have the luxury of eSATA or internal drives. My last edit machine had 4 internal drives: one for the OS, one for video capture, one for audio capture and one for render. Worked VERY well until drives started dropping like flies...

I edit HD off of a FW800 external all the time.
Does that make me the exception to the rule?

what is a green disk?

Doesn't that have an express card slot? If so, esata can connect to it.

Green disk are disk that use low power, the drawback being the lower performance mainly due to lower speed rotation. Since video editing usually requires above average performance and green disk are average or under, you can expect problems.

I’ve made due with FW400 in the distant past for DV – it was ok.

Now, for (sans highest setting) Cineform, HDV, AVCHD or DV, a simple second sata works well enough. Processor speed is more of an issue with these compressed formats than modern hard drive speeds. Most hard drive these days are more than fast enough. I’ll also mention that I don’t commonly work with multiple streams

For those who usually work with multiple streams, would a straightforward software raid work?

He's not entirely wrong, but he's not entirely right either.

Firewire will give you better performance than USB 2.0, and FireWire 800 (the new firewire) will give you even better performance than firewire. But eSATA gives you performance equivilant to an internal HD, which is why I prefer to use them.

You will experience slowdown, especially with AVCHD at those bitrates, on a USB 2.0 connection. USB 2.0 operates in bursts, and can't maintain sustained speeds; the problem is not reading one file at a time, but when you have multiple files that need to be read simultaniously. Like, for example, putting more than one video together in the same project. And any dissolve effect will need to read more than one video file at once.

USB 2.0 drives are still useful, however, if you're proxy editing OR you're willing to live with stuttery/slow playback during the editing phase. Vegas is actually pretty good about USB 2.0 drives, Final Cut Pro is not... for various reasons.

Honestly, you have a choice of enclosures out there, I'd go ahead and get one that can operate as EITHER USB 2.0 or eSATA. If USB 2.0 works for you, use that. If not, go with eSATA.

In my Experience with HDV and MXF files, USB is no good and 1394a (FW 400) can be problematic at times.

1394b (FW800) drives and eSATA are no problem at all. As to the why use FW800 over eSATA? In my case the main reason is daisy chaining. I can run 4 FW drives off my FW PCI 2.0 card(2 inputs) of mixed drive sizes with no issue. My eSATA card can only run 2 drives and they must be the exact same size.

I love the eSATA for content back-up and tend to use it as such. I have clients that provide me with content on FW800 drives for editing or capture and I can just chain them in without disabling any of my drives. If it were an eSATA drive they would have to provide me with one that is the exact same size and I would have to disable one of my drives to plugin their drive.

If you have a open 5.25" drive bay and a spare internal SATA connection, go with this kind of trayless removable drive system. It gives you virtually unlimited storage.

Trayless Removable Harddrive System

Tom's hardware just ran an interesting comparison of a 3 year old, 200G 7200RPM drive against a current, 1T, 5400 "green" drive. The newer drive seemed to be more capable on virtually every analysis, simple because technology continues to improve.

This has me debating new drives - I already have a new 500G for a new build, and it's very noticeably zippier, which leads me to suspect that the same would be true while pulling things such as AVCHD files off the disks - a faster drive might improve throughput enough to give better overall performance.

Optimizing a computer becomes an interesting task, as ALL the subsystems can affect performance, and you're only as fast as the slowest (bottleneck) subsystem... meaning CPU speed, FSB, Memory, hard disk and (not so much for video editing) video, each play a part.

I'm not too sure about anything some "computer store guy" says... unless they've got direct experience, it could be some secondhand hearsay from a web forum (and aside from DVi, we know how accurate THOSE are!). YMMV...

Another point to be aware of is that once you are using a file based video then the problem should be improved from DV or HDV and these would mainly have problems on capture. Once the files are on the hard drives editing will just have slowdowns etc but no losses. Issues will then arise on export to tape.
Only capture and export are critical that there is contiguous file transfer and where continuous drive throughput is important.
IF you are editing DV or HDV then the drive issues are real. IF you are editing AVCHD then drives are less of a problem but CPU power is VERY important and is the bottleneck. IF you are using an intermediate like Cineform or Canopus HQ the transfer rates will be much higher and thus drive speed is again important for editing( but you will not loose anything just editing preview quality). When you render your final file again drive speed is not important. This is purely a computational task.
Bottom line. For DV and HDV ( they are the same data rate) capture to a fast drive, any modern internal or a single external should be fine. Export to tape from a fast drive. These are the only times where realtime performance is critical. IF you are capturing HDV with realtime conversion to Cineform or Canopus HQ then drive speed and CPU power will be important.
As far as I am concerned it doesn't matter too much for AVCHD as this is a file copy process either from a hard drive in the camera ( which will be slower than all the hard drives on your PC) or from a memory card ( again likely slower than your hard drives)

Ron Evans

It's really about data density. For performance we want the largest capacity on the fewest platters. This setup moves the most data under the head per unit of time.

I would not buy "green" drives except for backup. The smallest drive I would buy today is 1TB. As soon as the 2TB drives get up to 7200 rpm I will buy those. Remember too that as drives get full they slow down, especially with raid. For the main edit drives I shoot for 2x capacity to keep under 50% full. OSX is good about using the outside fastest part of the drive first. Not sure what windows does.

Esata is just sata with better connectors. If an esata drive attached to a card is slower than internal drives it means the PCI card is cheap.