> Are you using MTD partitioning? -- does the Yaffs data start
> where you think it should, does uboot know about this?

No, I'm not using MTD partitioning for this NAND flash but I'm using 
partitioning for NOR flash (shouldn't have any impact here).

I copied that data to the NAND flash by first mounting the system on 
NFS, "eraseall /dev/mtd3", "mount -t yaffs /dev/mtd3 /mnt" and "cp -drf" 
the necessary files into /mnt.

~ # dmesg
...
Creating 1 MTD partitions on "K9F1208U0B":
0x00000000-0x04000000 : "Filesystems"
...
yaffs: dev is 32505859 name is "mtdblock3"
yaffs: Attempting MTD mount on 31.3, "mtdblock3"
VFS: Mounted root (yaffs filesystem).
...

 > If first 5 pages (0-9ff) are blank, then you can expect ECC check
 > failure on these pages.

No, the first 5 pages are not blank. They seemed to be valid data. The 
first 0x1B bytes are "..........Source............" in ascii

<First page data>
0x00000000: 03 00 00 00 01 00 00 00 ff ff 53 6f 75 72 63 65
0x00000010: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
0x00000020: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
0x00000030: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
0x00000040: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
0x00000050: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
0x00000060: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
0x00000070: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
0x00000080: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
0x00000090: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
0x000000a0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
0x000000b0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
0x000000c0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
0x000000d0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
0x000000e0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
0x000000f0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
0x00000100: 00 00 00 00 00 00 00 00 00 00 ff ff ed 41 00 00
0x00000110: 00 00 00 00 00 00 00 00 4a db b5 44 4a db b5 44
0x00000120: 4a db b5 44 ff ff ff ff ff ff ff ff ff ff ff ff
0x00000130: ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff
0x00000140: ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff
0x00000150: ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff
0x00000160: ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff
0x00000170: ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff
0x00000180: ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff
0x00000190: ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff
0x000001a0: ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff
0x000001b0: ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff
0x000001c0: ff ff ff ff ff ff ff ff ff ff ff ff 00 00 00 00
0x000001d0: ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff
0x000001e0: ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff
0x000001f0: ff ff ff ff ff ff ff ff 00 00 00 00 00 00 00 00
   OOB Data: 00 00 10 00 00 ff 05 01 ff 3f f3 f8 00 ff f3 ff