IMHO, forget Copy and Paste. Just start typing a new transaction. As you select the Payee Name, information about the previous check transaction to the same Payee should be retrieved to fill in the blanks.
Can you enter a write check straight out in the IRA? I doubt it but cannot chech it out right now. Because of taxation set up, I think only withdrawals are valid; not checks.
From C. D. Bales:You can create a WriteChk transaction from scratch in a Quicken IRA account.
You can't Paste transactions that deposit cash to, or withdraw cash from, a Quicken IRA account.
And you can't even Copy contribution transactions in an IRA account.