John writes: I am running out of free space on a 1 TB external hard drive that contains my iTunes library. I know it’s time to move to a larger drive but I’m hoping to spend as little as possible. My current drive is attached to my Mac via FireWire 800 but new drives with only USB 2.0 connections seem to be the least expensive.
Will a USB 2.0 connection be fast enough to stream my iTunes content to my Mac and other devices in my home, such as my Apple TV?
There are several different connectivity options available for Mac users today, and they each have pros and cons. In general USB 2.0 is the slowest interface but its efficacy will be determined by how you intend to use it.
In the case of an external iTunes library, USB 2.0 should be more than sufficient for most users. Access time will depend on the physical drive inside the external enclosure but once the data is flowing, most iTunes content will not saturate the USB 2.0 limit when streamed.
For example, a 1080p video is likely the largest file that current iTunes users would want to access from their computers or Apple TVs. The 2012 film Wreck-It Ralph, available on iTunes in 1080p HD with a 5.1 Dolby Digital audio track, has a peak bitrate of approximately 11.5 megabits per second. Looking at the table, above, it’s clear that this is safely accommodated by USB 2.0’s maximum theoretical bandwidth of 400 megabits per second.
Note: make sure to note the difference between bits and bytes. Interface transfer speeds are usually conveyed in megabits per second while real-world transfers are evaluated in terms of the more common megabytes per second. There are 8 bits in each byte, so USB 2.0's 400 megabit per second bandwidth equates to 50 megabytes per second.
The bitrate of video content will vary, and USB 2.0 speed is never going to reach the theoretical maximum, but even if the actual transfer speed was cut in half to 200 megabits per second, the interface could still comfortably handle iTunes-formatted 1080p video. Even less-compressed and higher bitrate Blu-ray video, at around 40 megabits per second, would work.
With this understanding, we can see that 720p resolution iTunes video, and audio content, which has substantially lower bitrates than video content, will also both work just fine.
There are some caveats, however. The above discussion examines only the experience for the user once the data is on the external drive and is being streamed to one computer or device. If you plan to modify your iTunes content by editing audio or video files from the external drive, or if you anticipate adding content regularly, a faster interface will speed up the process.
For example, the 1080p Wreck-It Ralph video is about 3.54 gigabytes in size. We copied it from our iMac’s internal SSD to a 2 terabyte USB 2.0 drive. The transfer took 109 seconds for an average transfer rate of 266 megabits per second (33.25 megabytes per second). For comparison, we then transferred the same file from the iMac to a LaCie Rugged external drive (via FireWire 800) and once more from the iMac to LaCie Little Big Disk (via Thunderbolt). The results can be seen in the chart, below:
The type of drive in an external enclosure and other system-specific factors will alter the exact transfer statistics, but the numbers above are generally what can be expected in a “best case” scenario.
Thunderbolt therefore offers a significant performance advantage but at an extremely increased cost while FireWire 800 provides a decent compromise between cost and performance. Whether the seconds or minutes saved on each file transferred are worth the cost of moving to a faster interface is a decision unique to each user.
Another factor, mentioned above, is the anticipated number of concurrent streams. If you plan to use just one or even two Macs or Apple TVs to stream content from the drive at the same time, USB 2.0 will likely be sufficient. If you’ve built up an extensive network of Apple devices in your home and plan to stream concurrently to several Apple TVs, Macs, and iDevices, however, you’ll likely want to choose both a faster interface and a faster drive to accommodate the multiple read requests that such as setup would demand.
In summary, if you don’t mind longer initial transfer times, don’t plan to perform extensive modifications to the files once they are on the external drive, and only plan to stream concurrently to one or two devices, a USB 2.0 external hard drive interface will likely be sufficient for housing an iTunes library. If time is a factor, investing in FireWire 800 or Thunderbolt may be a better, albeit more expensive, option.