This might help you:
When using its default binary method of storage, cpio cannot save files larger than two gigabytes in size in an archive. When storing archives in ASCII (because of -c), cpio cannot save files larger than eight gigabytes in size in an archive. However, other programs that extract files from cpio archives may be unable to properly extract files larger than two gigabytes and cpio issues a warning to that effect when you attempt to save such a file. "
You might want to try to compress the data first:
# find / | cpio -ocvB | gzip >/dev/rct0
I use the above code all the time to back up my system, but I write it to a file and then burn the file to a CD/DVD:
# find /usr | cpio -ocv | gzip >usr.cgz
Or just you could re-direct standard out to the device:
# find / -depth -print | cpio -ocvB >/dev/rct0
Are you really gaining anything with changing the buffer size (-B) from 512???