Usually it's better to place data file, log and temp on a different disk than the operative system: in this case even if OS perform any heavy operation, the database doesn't slow down.
Of course there's some kind of risk using a non-raid drive.
Absolutely it affects the performance of the system. But it's all relative. Spend some time reviewing the many post you will find on the subject. But in general, a default (everything on C) may be adequate for a tiny demo system but little else.
Thank you for all your advice, experts! But if we don't have many users (around 20), is it very necessary we install the SQL Server using the Raid configuration? Thank you!
In my opinion 20 users is not a little quantity at all: a Raid0 stripe can improve the performance for sure, but I suggest a Raid10 to prevent data loss.
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Of course there's some kind of risk using a non-raid drive.