C# - SQL - LINQ Query

Jane Doe has the right idea. Just to add an explanation, it seems from your vague description you wrote this:

for (int i = 0; i < driveList.Count; i++)
var q2 =
    from sd in db.ServerDrives
    where sd.DriveLetter == driveList[i].Name
    select sd;
ServerDrive existingServerDrives = q.First();
existingServerDrives.FreeSpace = driveList[i].FreeSpace;
//..
db.SubmitChanges();

This is interpreted as follows:

for (int i = 0; i < driveList.Count; i++)
{
    var q2 =
        from sd in db.ServerDrives
        where sd.DriveLetter == driveList[i].Name
        select sd;
}

ServerDrive existingServerDrives = q.First();
existingServerDrives.FreeSpace = driveList[i].FreeSpace;
//..
db.SubmitChanges();

The variable i is only valid inside the loop body. The solution is to put the curly braces around all the code you wish to run in the loop and also you need to change q to q2.

for (int i = 0; i < driveList.Count; i++)
{
    var q2 =
        from sd in db.ServerDrives
        where sd.DriveLetter == driveList[i].Name
        select sd;

    ServerDrive existingServerDrives = q2.First();
    existingServerDrives.FreeSpace = driveList[i].FreeSpace;
    //..
    db.SubmitChanges();

}

Note also that this is not the most efficient way to update multiple rows because you are performing two database requests per update.

Update: please read clarification in the end of answer for details about closure handling.

First things first. This code:

    for (int i = 0; i < driveList.Count; i++)
    {
        var q =
            from sd in db.ServerDrives
            where sd.DriveLetter == driveList[i].Name
            select sd;
        //...
   }

won't work irregardless of curly braces. The reason for that is closure of i (which is masked out by funny LINQ syntax). The easiest way to fix it is to store i value (or referenced value) in separate variable.

    for (int i = 0; i < driveList.Count; i++)
    {
        var j = i;
        var q =
            from sd in db.ServerDrives
            where sd.DriveLetter == driveList[j].Name
            select sd;
        //...
    }

This will work, but it looks loosy. Better:

    for (int i = 0; i < driveList.Count; i++)
    {
        var drive = driveList[i];
        var q =
            from sd in db.ServerDrives
            where sd.DriveLetter == drive.Name
            select sd;
        ServerDrive existingServerDrives = q.First();
        existingServerDrives.FreeSpace = drive.FreeSpace;
        //..
        db.SubmitChanges();
    }

But why would you need loop in first place? Why not foreach?

    foreach (var drive in driveList)
    {
        var q =
            from sd in db.ServerDrives
            where sd.DriveLetter == drive.Name
            select sd;
        ServerDrive existingServerDrives = q.First();
        existingServerDrives.FreeSpace = drive.FreeSpace;
        //..
        db.SubmitChanges();
    }

But this won't work too, be careful. Same trouble with closures. Solution is easy:

    foreach (var d in driveList)
    {
        var drive = d;
        var q =
            from sd in db.ServerDrives
            where sd.DriveLetter == drive.Name
            select sd;
        ServerDrive existingServerDrives = q.First();
        existingServerDrives.FreeSpace = drive.FreeSpace;
        //..
        db.SubmitChanges();
    }

But of course such things are better with projections:

    foreach (var d in driveList.Select(d => new {freeSpace = d.FreeSpace, existingServerDrives = db.ServerDrives.First(sd => sd.DriveLetter == d.Name)}))
    {
        d.existingServerDrives.FreeSpace = d.freeSpace;
        //..
        db.SubmitChanges();
    }

Clarification: In fact, I am not completely right about "will not work" part. It will if driveList is IQueryable, but it won't if it is IEnumerable (because the former uses expressions, and the latter - delegates). But because syntax is same, it is all too easy to make this error, so you still should not use loop variables inside lambda expressions irregardless of their real type.