bool IsDriveReady(char drive)
{
bool out;
WORD OldErrorMode = SetErrorMode(SEM_FAILCRITICALERRORS);
AnsiString path = AnsiString(drive)+":\\";
out = (SetCurrentDirectory(path.c_str()) != 0);
SetErrorMode(OldErrorMode);
return out;
}

if (IsDriveReady('a'))
        {
        ShowMessage("Дискета вставлена");
        }