为何提示说文件不存在？

[honey_001]

帮我看看下面的代码是哪儿错了？
procedure TForm3.BitBtn5Click(Sender: TObject);
var
sd,s1,s2,s:string;
begin
//panel3.Visible:=true;
sd:=copy(label2.caption,7,2);
dm3.ADOquery2.Close;
dm3.adoquery2.sql.Clear;
dm3.ADOQuery2.sql.Add('select * from detailed_table where adata='+quotedstr(sd)+'order by uid');
dm3.ADOquery2.open;
dbsumlist1.DataSet:=dm3.adoquery2;
str1:=currtostr(dbsumlist1.SumCollection.Items[0].SumValue);
str2:=currtostr(dbsumlist1.SumCollection.Items[1].SumValue);
str3:=currtostr(dbsumlist1.SumCollection.Items[2].SumValue);
str4:=currtostr(dbsumlist1.SumCollection.Items[3].SumValue);
str5:=currtostr(dbsumlist1.SumCollection.Items[4].SumValue);
str6:=currtostr(dbsumlist1.SumCollection.Items[5].SumValue);
str7:=currtostr(dbsumlist1.SumCollection.Items[6].SumValue);
str8:=currtostr(dbsumlist1.SumCollection.Items[7].SumValue);
str9:=currtostr(dbsumlist1.SumCollection.Items[8].SumValue);
str10:=currtostr(dbsumlist1.SumCollection.Items[9].SumValue);
str11:=currtostr(dbsumlist1.SumCollection.Items[10].SumValue);
str12:=currtostr(dbsumlist1.SumCollection.Items[11].SumValue);
str13:=currtostr(dbsumlist1.SumCollection.Items[12].SumValue);
str14:=currtostr(dbsumlist1.SumCollection.Items[13].SumValue);
str15:=currtostr(dbsumlist1.SumCollection.Items[14].SumValue);
s1:=copy(dm3.ADOTable3.Fields[1].AsString,1,4)+copy(label2.Caption,1,10);
if radiobutton1.Checked then
assignfile(copytxt,'c:\'+s1+'.txt')
else
assignfile(copytxt,'a:\'+s1+'.txt');
s2:='c:\'+s1+'.txt';
rewrite(copytxt);
try
begin
s:='编制单位:'+dm3.ADOTable3.Fields[1].AsString;
writeln(copytxt,s);
writeln(copytxt,str1);
writeln(copytxt,str2);
writeln(copytxt,str3);
writeln(copytxt,str4);
writeln(copytxt,str5);
writeln(copytxt,str6);
writeln(copytxt,str7);
writeln(copytxt,str8);
writeln(copytxt,str9);
writeln(copytxt,str10);
writeln(copytxt,str11);
writeln(copytxt,str12);
writeln(copytxt,str13);
writeln(copytxt,str14);
writeln(copytxt,str15);
CloseFile(copytxt);
end;
savedialog1.FileName:=s1;
savedialog1.Filter:='txt';
if saveDialog1.Execute then
begin
AssignFile(copytxt, saveDialog1.FileName);
Append(copytxt);
Flush(copytxt);
CloseFile(copytxt);
end;

finally
closefile(copytxt);
end;
end;



if saveDialog1.Execute then
begin
AssignFile(copytxt, saveDialog1.FileName);
Append(copytxt);
Flush(copytxt);
CloseFile(copytxt);
end;
Append(copytxt);提示文件不存在。我明明已经创建了文件。