In Final Cut Pro, hard drives designated as "scratch disks," store captured and rendered media for editing, playback, and output. Final Cut Pro may drop frames during capture, playback, or output if you have set the scratch disk location to be a network hard drive that is connected to your computer via Ethernet.
Scratch disk performance is a critical aspect of your editing system. The available data rate of your disks must match or exceed the video format you are using.
Even in cases where the data rate of the Ethernet standard exceeds the data rate required by the video format you are using, dropped frames can occur. This can happen because Final Cut Pro will often need to access more than one stream of audio/video media at a time. Additionally, the available bandwidth on an Ethernet network must accommodate the overhead required by networking protocols, as well as any traffic on the network generated by other users or by automated processes. In most professional workflows, Ethernet will not provide sufficiently high sustainable bandwidth for the smooth transfer of video and audio data needed by Final Cut Pro. (This includes "wired" Ethernet formats, as well as wireless variants, i.e., 802.11a, b, g and n.)
Use hard drives installed inside your computer, or external drives that are directly connected to your computer via FireWire or SCSI. Multi-user or high-capacity environments may require a RAID or SAN that is directly connected to the computer via a Fibre Channel connection.
For more information about setting scratch disks, refer to the Final Cut Pro User Manual.