If you're using Backup II v1.1, we suggest you go back to ProDOS 1.2 instead of 1.4, to have fewer changes in the operating system.
By not using ProDOS 1.4, all you'll be giving up is support for the Apple IIGS. ProDOS versions 1.3 and 1.2 work correctly on other systems, just not on the Apple IIGS.
Note: This incompatibility has been solved in the current version of Backup II (v1.1.1), which will function perfectly well with ProDOS 1.4, and which also solves some other problems found in Backup II v1.1, such as failures of the "restore" function.