Depends on the drive and how full it is. But you should be able to playback 1 stream of ProRes HD without too much issue on a single disk setup - but I don't recommend it.
Time to think about a small RAID array for the media drive. Basic 2 drive RAID 0 would work but you might want to look at the newer hardware RAID 5 boxes out there with 4 or more HDDs.
Alternatively an SSD or SSD RAID (if you have the money).
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...the ideal setup for freelance editing at home
What are your requirements? What do you deliver? How often are you doing it? What turn-around do you expect?
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