If the two drives you mentioned do not work when connected to a pc, then only a professional data recovery company can help you, unless you have the files you need in a backup.
But if they should work, please check the mainboard of your LaCie5Big Network2.
I am currently dealing with a similar problem and noticed that the 2.5V 1F supercap next to the marvel controller had leaked.
This probably caused the raid5 to fail.
As all my discs were still working when connected to my pc, I was able to backup all the drives to .vhd images with winimage (free 30 day test version),
and I managed to access the xfs raid with "recovery explorer raid".
I am currently in the process of backing up all my data to a new drive.
(you basically need the free space of the drive capacity + the raid size)
however if your not willing to spend money on software, these guides that I just found would probably got me the data back as well and sureley saved me a lot of time.
I had tried to fix the virtual raid5 array with mdadm first, but did something wrong and because I didn't know there was an option to make an overlay I had to make new vhd images again.
https://raid.wiki.kernel.org/index.php/RAID_Recoveryhttps://raid.wiki.kernel.org/index.php/ ... tware_RAIDfor the LaCie5Big N2:
I removed the defective supercap and cleaned the board and the contacts.
I also replaced all the electrolytic capacitors on the mainboard of the 5big.
(Don't try this yourself unless you have soldering equipment that works with multilayer pcb's)
As soon as my data is save, I will power up the NAS and see if I can get it working again.