Well, it's not completely true, but close. On a Lacie the OS is on the disk. The 5Big2 cannot boot the OS of the 5Big (that's why you couldn't connect), so at least the OS has to be exchanged.
Unfortunately the default way an OS is installed causes the datapartitions to be reformatted. Yet it should be possible to install the OS manually, and preserve the data. But you need some Linux knowledge, no fear for the command line, and a serial connection to your box. Some more info here
Did you get the 5Big2 without disks? How are you supposed to install your old disks?
Unfortunately I have used my 5Big V1 in Raid 5 mode, so I don't have a chance to read else where.
Fortunately Lacie uses default Linux software raid. So you can assemble your array when you can connect 4 disks to a Linux system. (Any PC booted from a Linux Live CD or -USB stick will do). The connection can be done using the internal SATA ports, ESATA ports, or using USB to SATA converters. You can also use a combination of these connections. (You could also connect the disks to 2-4 different computers, and share the partitions over a network as Network Block Devices. Oh yeah, Linux is a great OS)