Playback and Recording : Record Setup : Working with Hard Drives for Recording : Recording to the System Volume

Recording to the System Volume
Although Pro Tools lets you record to your system volume, this is generally not recommended. Performance for audio recording and playback on system drives is not as good as on non-system hard drives.
Record to system drives only when absolutely necessary, such as if your computer system has only one hard drive, or if your other hard drives are completely full.
By default, the system volume is not included in Round Robin Allocation (regardless of volume designation in the Workspace browser). To include the System Volume in Round Robin Allocations, see Allocating Audio Drives in Your System.
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If you have a Pro Tools system with multiple drives, and you intend to record multiple tracks simultaneously, you may want to designate the System drive as a Playback only or Transfer only drive for optimal performance.