Then I noticed in the register of my checking account that there was no "TXFR" under the "Num" column, even though this was a transfer between accounts. So I updated it and - poof - the balance of my fund account drops to 0, even though the balance of my checking account correctly reflects that cash was transferred out. So I went through the whole thing again, deleting the account and creating it, and then entered text into the notes fields of the transaction "Do not update Num column." Poof - the balance of my fund account drops to 0. Any change I make to the linked transaction causes the problem. When I look in the mutual fund account, I still see the transaction there (including the note text that I added from the checking account) but the quantity of shares and cost per share are zero'd out.
What on earth am I doing wrong? I can live with the empty "Num" column, but I cannot live with a situation that causes transactions to get wiped out without notice. Someday I will accidentally type in the notes field and I do not want to loose the transaction!