I highly recommend using either /system/build.prop.backup or /sdcard/build.prop.backup as custom paths. /storage/emulated/0/foo/bar/foo.bar will fail! If you encounter issues when using backups, please try using one of the recommend paths first before reporting. Some paths vary across different devices/ROMs.
After all, BACKUP=y is probably the safest and easiest way to safe your original build.prop file.