From the examples I can easily read the entire file with myFile.read()
No, you can't. read() reads one character. There are other overloads that take arguments that allow you to read more than one character. If you have fixed length records, you could read an entire record in one shot.
while (myFile.available()) {
x = (myFile.read());
}
The parentheses around myFile.read() are not needed. The result of this while loop is that the last character from the file is stored in x. That hardly seems like what you want to do.
while (myFile.available()) {
y = (myFile.read());
}
Similarly, the last character from this file is stored in y. Again, not likely what you want.
I figured I could just put one entry per file.
So, you plan to have 200 files? Not a good plan, if speed is any kind of issue.