When you create a temporary assembly do not use a start asm that has datum planes. Place the asm you want to mirror in the temp asm and use Component > Create > subasm > Mirror. Give it a name and choose which components that you want to reuse and ones that you want to be new.
WHEN YOU SELECT A PLANE TO MIRROR ABOUT ONLY USE ONE FROM THE ORIGINAL ASM. IF YOU MIRROR ABOUT A PLANE IN THE TOP LEVEL TEMP ASM YOU WILL CREATE AN UNWANTED RELATIONSHIP. (SORRY FOR CAPS)
Another tip is that you can save only the new mirrored asm without saving the temp asm when you select File > save and type in the name of the new mirrired asm instead of accepting the default temp asm name in the message line. Only the sub asm will be saved and you can wipe out ram and not have to manually delete the temp asm.