One problem you could have in trying to copy files via a PC would be that the PC would not recognise them. There is also the possibility they might be corrupted in the process although if you have a back up that should not be a worry.

That said I'm guessing it would take just as long as restoring from a back up so, if it is essential both drives be the same size, then my earlier suggestion would not work. The RAID system might do all sorts of odd things so it really does look like you would be best to restore from a back up to the new bigger drives.

What you are attempting would appear the most economic way of increasing capacity if you are sure you want to retain the dual HDD RAID system. But it does beg the question as to whether having that dual HDD RAID system might be overkill & there could be enough security in regular backups.

John