A two-phase process FlashCopy operation is provided that can be used to aid in the formation of consistency groups across multiple storage control units. In the first phase, preparations to create a new consistency group are made "revertible" by write-inhibiting the source volumes through "Establish-FlashCopy-revertible" commands. If the preparation of any volume within the consistency group fails, a "Withdraw-FlashCopy-revert" command may be executed, thereby causing a retention of the prior FlashCopy point-in-time copy. In the second phase, executed if all preparations are successful, a "Withdraw-FlashCopy-commit" command may be executed to remove all write-inhibit indicators, complete the creation of the new FlashCopy point-in-time copy and secure the new consistency group. Write requests to the FlashCopy source volumes may then be received and processed without risking corruption of the new consistency group on the Flashcopy target volumes.