[Updated Oct 4, 2019]
Recording audio to your system drive past 2 inputs at a time is general considered a bad idea, using dedicated empty drives for audio data and not the system drive is the best choice for quality multi-track audio.
Your operating system uses the main drive for programs and for caching the main drive for additional memory provided your system doesn't have enough to process all of your applications memory requirements.
A side note about System Memory in relation to hard drive performance. For example a modern OS such as Windows 7, 8.1, 10 or Mac OS X with 4 GB of RAM may actually be too small. These OS's require 2 GB to run, plus 2 GB for the application, any additional memory programs need will use the Hard Drive for caching. It's always a good idea to increase the system memory to it's maximum potential to reduce or eliminate caching or "Disk Too Slow" type errors that may occur as a result of low system resources.
What I'm discussing here applies to any multi-channel DAW including Studio One. Other programs like Pro Tools, Sonar or Nuendo may actually halt your transport due to error checking that checks for dropped samples during the disk writing process.
For multi-channel audio recordings especially ones that utilize programs like Capture where as many as 32 inputs for single mixer (or 64 for cascaded mixers) are being recorded at once, the data path to your computer must be very fast. You'll need to use a 1394 (FireWire), eSATA, USB 3.0 or Thunderbolt.
Consider what you are recording 16.4.2 = 16 inputs @ 44.1KHz = 5MB / minute per input = 80 MB a minute or = 1.4MB / Sec being written to 16 individual file paths. The number goes up for more inputs and even harder on system requirements for 24 bit or and/or 48Khz.
We strongly recommend against USB 2.0 for audio data, even with a fast hard drive (see next paragraph), if you have too many samples (or audio tracks), it may choke and display a "Disk Too Slow" Error. USB 2.0 works in burst mode, and does not use a sustained data rate for large amounts of data. 1394 and Thunderbolt on the other hand will use a sustained data rate and is better for audio. USB 3.0 has also been seen recently as a good choice as it can handle the data bandwidth for multi-channel audio in large numbers.
The hard drive must be at least 7200RPM or faster, many inexpensive external Hard drives found in USB 2.0 and USB 3.0 cases only run at 4200 or 5400 rpm which makes them too slow for real time audio access. Many Core 2 Duo and entry to mid-level recent model and current (Mac or PC) Intel i-Series and (PC) AMD A-Series laptops all have these drives internally. These 4200RPM and 5400RPM drives are most of the time the culprit for "Disk Too Slow" Errors or as we've seen recently errors where cascaded mixers capture one mixer audio channels correctly, but not the other.
Thunderbolt enclosures recently came market as of Q1/2014, and your computer must have Thunderbolt on the motherboard or built into the laptop as there are no expansion cards for that technology.
If you run into any trouble using external drives, then consider upgrading your internal drives on your system to 7200 RPM or faster drives. Even a low priced SSD albeit smaller size will perform better than an equally priced 5400 or 4200RPM HDD. If you're on a laptop, consider ditching the DVD-ROM drive for a 2nd HDD/SSD. There are after market solutions for Apple Mac Book / Pro's and for PC's.
The Avastor HDX Pro dual USB-C (USB 3.1 Gen2) port drives have been tested extensively with our Studio Series interfaces, StudioLive Mixers and Thunderbolt series products on Mac and Windows systems with great success for both Studio and Live situations. https://oyendigital.com/hard-drives/store/HDX-C.html
MacBook Pro users who have a DVD drive in their systems can consider this as an option:
PC Laptop users, usually only need to look as far as their vendor's website or a Google search to find something similar for their systems.
Most importantly if you save any sessions or songs with sounds used on that drive, you'll need to keep that drive connected the next time you load that session, or Studio One will throw an error saying it can't find it. You'll need to navigate to the directory or drive where the files are located.
If you're looking for a quality external hard drive solution, go with the best, we recommend Avastor drives. These guys used to be former Ampex and Memorex employees, they know audio recording like no other. You can find them on the web at: http://www.avastor.com