root@OpenWrt:/# cd /tmp
root@OpenWrt:/tmp# wget http://download.openwrt.net/ar71xx/tmp/caldata/part_01
Connecting to download.openwrt.net (178.72.240.58:80)
part_01              100% |*******************************| 65440  --:--:-- ETA
root@OpenWrt:/tmp# wget http://download.openwrt.net/ar71xx/tmp/caldata/part_02
Connecting to download.openwrt.net (178.72.240.58:80)
part_02              100% |*******************************|     3  --:--:-- ETA
root@OpenWrt:/tmp# wget http://download.openwrt.net/ar71xx/tmp/caldata/part_03
Connecting to download.openwrt.net (178.72.240.58:80)
part_03              100% |*******************************|    59  --:--:-- ETA
root@OpenWrt:/tmp# echo -n 00:24:01:E7:75:BC > mac1
root@OpenWrt:/tmp# echo -n 00:24:01:E7:75:BD > mac2
root@OpenWrt:/tmp# cat part_01 mac1 part_02 mac2 part_03 > caldata
root@OpenWrt:/tmp#  ls -l caldata
-rw-r--r--    1 root     root        65536 Jan  1 00:11 caldata
root@OpenWrt:/tmp# mtd unlock caldata
Unlocking caldata ...
root@OpenWrt:/tmp# mtd -e caldata write caldata caldata
Unlocking caldata ...
Erasing caldata ...
Writing from caldata to caldata ...     
root@OpenWrt:/tmp# 

 BusyBox v1.19.4 (2013-03-14 11:28:31 UTC) built-in shell (ash)
Enter 'help' for a list of built-in commands.

  _______                     ________        __
 |       |.-----.-----.-----.|  |  |  |.----.|  |_
 |   -   ||  _  |  -__|     ||  |  |  ||   _||   _|
 |_______||   __|_____|__|__||________||__|  |____|
          |__| W I R E L E S S   F R E E D O M
 -----------------------------------------------------
 ATTITUDE ADJUSTMENT (12.09, r36088)
 -----------------------------------------------------
  * 1/4 oz Vodka      Pour all ingredients into mixing
  * 1/4 oz Gin        tin with ice, strain into glass.
  * 1/4 oz Amaretto
  * 1/4 oz Triple sec
  * 1/4 oz Peach schnapps
  * 1/4 oz Sour mix
  * 1 splash Cranberry juice
 -----------------------------------------------------
root@OpenWrt:~# cd /tmp
root@OpenWrt:/tmp# wget http://downloads.openwrt.org/attitude_adjustment/12.09/a
r71xx/generic/openwrt-ar71xx-generic-dir-825-b1-fat-jffs2-sysupgrade.bin
Connecting to downloads.openwrt.org (78.24.191.177:80)
openwrt-ar71xx-gener 100% |*******************************|  3968k  0:00:00 ETA
root@OpenWrt:/tmp# sysupgrade -i openwrt-ar71xx-generic-dir-825-b1-fat-jffs2-sys
upgrade.bin
Do you have a backup of the caldata partition? (y/N): yes
Keep config files over reflash (Y/n): n
Sending TERM to remaining processes ... uhttpd dnsmasq ntpd syslogd klogd hotplug2 ubusd netifd 
Sending KILL to remaining processes ... uhttpd 
Switching to ramdisk...
Performing system upgrade...
calibration data mismatch caldata:b419fc0c9dc4df2d101c46005871ef2b caldata_copy:a294ae924ced36ac2257f6f06902c84f
unable to backup calibration data
@OpenWrt:/tmp# 

 cat /proc/mtd
dev:    size   erasesize  name
mtd0: 00040000 00010000 "uboot"
mtd1: 00010000 00010000 "config"
mtd2: 00610000 00010000 "firmware"
mtd3: 00125aad 00010000 "kernel"
mtd4: 004ea553 00010000 "rootfs"
mtd5: 00280000 00010000 "rootfs_data"
mtd6: 00010000 00010000 "caldata"
mtd7: 00190000 00010000 "unknown"
mtd8: 00010000 00010000 "caldata_copy"                                                                                                                               root@OpenWrt:/tmp# dd if=/dev/mdtblock mtd6 of=caldata
BusyBox v1.23.2 (2015-09-16 20:39:07 CEST) multi-call binary.

Usage: dd [if=FILE] [of=FILE] [ibs=N] [obs=N] [bs=N] [count=N] [skip=N]
    [seek=N] [conv=notrunc|noerror|sync|fsync]

Copy a file with converting and formatting

    if=FILE        Read from FILE instead of stdin
    of=FILE        Write to FILE instead of stdout
    bs=N        Read and write N bytes at a time
    ibs=N        Read N bytes at a time
    obs=N        Write N bytes at a time
    count=N        Copy only N input blocks
    skip=N        Skip N input blocks
    seek=N        Skip N output blocks
    conv=notrunc    Don't truncate output file
    conv=noerror    Continue after read errors
    conv=sync    Pad blocks with zeros
    conv=fsync    Physically write data out before finishing
    conv=swab    Swap every pair of bytes

N may be suffixed by c (1), w (2), b (512), kD (1000), k (1024), MD, M, GD, G

root@OpenWrt:/tmp# mtd unlock caldata
Could not open mtd device: caldata
Could not open mtd device: caldata
root@OpenWrt:/tmp# mtd -e caldata write caldata caldata
Couldn't open image file: caldata!
root@OpenWrt:/tmp# 

 root@OpenWrt:/tmp# dd if=/dev/mdtblock8 of=caldata
dd: can't open '/dev/mdtblock8': No such file or directory
root@OpenWrt:/tmp# 

ls /dev/mtd*
/dev/mtd0       /dev/mtd3       /dev/mtd6       /dev/mtdblock0  /dev/mtdblock6
/dev/mtd0ro     /dev/mtd3ro     /dev/mtd6ro     /dev/mtdblock1  /dev/mtdblock7
/dev/mtd1       /dev/mtd4       /dev/mtd7       /dev/mtdblock2  /dev/mtdblock8
/dev/mtd1ro     /dev/mtd4ro     /dev/mtd7ro     /dev/mtdblock3
/dev/mtd2       /dev/mtd5       /dev/mtd8       /dev/mtdblock4
/dev/mtd2ro     /dev/mtd5ro     /dev/mtd8ro     /dev/mtdblock5 

root@OpenWrt:/tmp# dd if=/dev/mtdblock8 of=caldata
128+0 records in
128+0 records out

mtd unlock caldata
mtd -e caldata write caldata caldata