Saving WNDR3700v4 calibration data

I'm trying to save my WNDR3700v4's calibration data just in case I ever need more than 16 MB flash available. When downloading caldata_backup via Luci, I'm offered a download of 256 kB, but when saved the file always has 0B.

Does this mean the backup tool can't cope with the NAND flash (see the warning about flashing calibration data), or Is my calibration data gone?

I do think I have only used sysupgrade after the first installation of OpenWRT on this router because I still see only 16 MB flash. The first release I installed was one of the first that supported this router.

Did you try using dd on the command line directly?

I have never seen an option to backup any system partitions in LuCI altogether (all I ever use is the configuration backup functionality and the occasional firmware upgrade).

dd if=/dev/mtd10ro worked if that is the correct device.

In od -x I see many lines containing 00 or FF; can this still be valid calibration data?

"Save mtdblock contents" is two paragraphs down from "Backup" on the Luci page.

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Blocks of zeroes or FFs are generally empty space. Most caldata partitions are rather sparse. On an Atheros system the data related to wifi usually starts at 0x1000.

Thanks to both of you, I'll keep the file I got from /dev/mtd10ro.

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