Yes. You can move hard drives between Macintosh computers running System 6
and System 7. When a hard disk is moved from System 6 to System 7, you
will see a dialog box when you start stating that "This disk is being
updated for new system software". This process does not affect any data on
your disk. Rather, it means the system is creating a new System 7 Desktop
file on the hard disk. Afterwards, you may use your hard disk as usual.
When a hard disk is moved from System 7 back to System 6, you will notice a
few differences under System 6. First, you should rebuild the Desktop of
the hard disk by holding down the Command and Option keys, while starting the
Finder. If you don't do this, the first time you go back to System 6, some
icons may not appear properly.
You may also notice two new folders which appear on your hard disk under
System 6 which were not present under System 7. The first of these is the
Desktop folder which will contain any items which you placed on the desktop
under System 7. The second folder you will see under System 6 is the Trash
folder. This folder will contain any items which were dragged to the Trash
(but not emptied) under System 7.
Moving a disk between System 6 and System 7 is safe and will not damage any
data.
Note: Locked media created under System 7 cannot be mounted under System 6
until they are unlocked.