HD Backup is incompatible with System 7, but there are a number of workarounds if you need to restore an HD Backup set of data onto a volume.
Workaround One
You can start up from a volume that has System 6 on it, such as a second hard disk or floppy disk. This works even if you have only a single floppy drive and no extra hard disk (although you'll be swapping floppy disks a lot). To start from the other drive, insert a System 6 floppy disk before startup, or choose the other hard disk using the Startup Disk control panel.
Workaround Two
Another workaround is to pull individual files from floppy disks in the backup set onto the hard disk. This works best if there are a few files that need to be restored, but not the entire backup set. You may have difficulty if HD Backup split large files across multiple floppy disks.
Workaround Three
The most drastic workaround is to reinstall System 6 on the hard disk and then run HD Backup. Note that this may be a problematic procedure in itself, and the details of that procedure aren't covered in this article. Specifically, most new Macintosh models require System 7 to operate and would not be able to move back down to System 6.