It would appear that there might possibly be 'a problem' with this NAS, on more of a 'firmware' or 'hardware' level, or so I suspect.
When resetting the network access to factory defaults, all files on the hard disks are fully available (Volume_1 is given 'R/W' access to all users) and the 15 files that were not accessible can now be assigned the necessary access rights (R/W).
But this can only be accomplished when there are no user accounts or if there is a single user account. When more than a single user account is added, all 15 files that were seemingly 'locked' are once again 'locked' and cannot be accessed or copied to another location, so I must presume this is a 'failure' on some part of the emedded Linux OS.
Having re-established some level of 'control' over the entire NAS file structure finally, we've decided to 'move' the entire contents of the file system to a 'spare' Windows XP box and simply access these via a 'peer-to-peer' Windows network - for now.
When I have the opportunity, I will install new hard disks (I 'suspect' one of the two drives that are now in RAID-1 configuration - and they are decidely too small, now, also ... 160 GB isn't what it once was, by today's considerations), and then 're-flash' the firmware with these new totally empty hard disks - and then see if this is still 'an issue'...