[Transformer TF300T Q&A, Help & Troubleshooting] neal121:
I sense a load of fun with this since I have yet to have a smooth time with rooting (well it is rooted) and getting things to work and now this error. (Other devices ive rooted and had clockwork work perfectly but anything with Asus label always screams - I will be a pain to get to work!)

CWM gives an error when backing up, here is the log.

Starting recovery on Fri Jan 11 00:24:55 2013
can't open /dev/tty0: No such file or directory
framebuffer: fd 3 (1280 x 800)
CWM-based Recovery v6.0.2.3
recovery filesystem table
=========================
0 /tmp ramdisk (null) (null) 0
1 /system ext4 /dev/block/mmcblk0p1 (null) 0
2 /cache ext4 /dev/block/mmcblk0p2 (null) 0
3 /misc emmc /dev/block/mmcblk0p3 (null) 0
4 /boot emmc boot (null) 0
5 /recovery emmc recovery (null) 0
6 /staging emmc /dev/block/mmcblk0p4 (null) 0
7 /btmac vfat /dev/block/mmcblk0p5 (null) 0
8 /data ext4 /dev/block/mmcblk0p8 (null) -32768
9 /emmc datamedia /dev/null (null) 0
10 /sdcard vfat /dev/block/mmcblk1p1 /dev/block/mmcblk1 0

W:Unable to get recovery.fstab info for /datadata during fstab generation!
W:Unable to get recovery.fstab info for /sd-ext during fstab generation!
I:Completed outputting fstab.
I:Processing arguments.
I:Checking arguments.
I:device_recovery_start()
Command: "/sbin/recovery"

ro.boot.serialno=015d2bc2843bfc03
ro.boot.commchip_id=0
ro.boot.productid=0x02
ro.boot.carrier=wifi-only
ro.serialno=015d2bc2843bfc03
ro.bootmode=unknown
ro.baseband=unknown
ro.bootloader=unknown
ro.hardware=cardhu
ro.revision=0
ro.emmc=0
ro.boot.emmc=0
ro.factorytest=0
ro.secure=1
ro.allow.mock.location=0
ro.debuggable=1
persist.sys.usb.config=mtp,adb
ro.build.id=JOP40D
ro.build.display.id=cm_tf300t-userdebug 4.2.1 JOP40D eng.xplodwild.20121218.214839 test-keys
ro.build.version.incremental=eng.xplodwild.20121218.214839
ro.build.version.sdk=17
ro.build.version.codename=REL
ro.build.version.release=4.2.1
ro.build.date=Tue Dec 18 21:49:07 UTC 2012
ro.build.date.utc=0
ro.build.type=userdebug
ro.build.user=xplodwild
ro.build.host=ribs
ro.build.tags=test-keys
ro.product.model=Transformer Pad TF300T
ro.product.brand=asus
ro.product.name=EeePad
ro.product.device=tf300t
ro.product.board=cardhu
ro.product.cpu.abi=armeabi-v7a
ro.product.cpu.abi2=armeabi
ro.product.manufacturer=asus
ro.product.locale.language=en
ro.product.locale.region=US
ro.wifi.channels=
ro.board.platform=tegra
ro.build.product=tf300t
ro.build.description=WW_epad-user 4.0.3 IML74K WW_epad-9.4.3.30-20120604 release-keys
ro.build.fingerprint=asus/WW_epad/EeePad:4.0.3/IML74K/WW_epad-9.4.3.30-20120604:user/release-keys
ro.build.characteristics=tablet
ro.cm.device=tf300t
ro.carrier=wifi-only
ro.sf.lcd_density=160
ro.com.android.dateformat=MM-dd-yyyy
wifi.interface=wlan0
wifi.supplicant_scan_interval=15
ro.opengles.version=131072
persist.tegra.nvmmlite=1
ro.epad.model=TF300T
dalvik.vm.heapstartsize=5m
dalvik.vm.heapgrowthlimit=48m
dalvik.vm.heapsize=256m
dalvik.vm.heaptargetutilization=0.75
dalvik.vm.heapminfree=512k
dalvik.vm.heapmaxfree=2m
ro.rommanager.developerid=cyanogenmod
keyguard.no_require_sim=true
ro.com.google.clientidbase=android-google
ro.com.android.wifi-watchlist=GoogleGuest
ro.setupwizard.enterprise_mode=1
ro.com.android.dataroaming=false
ro.cm.version=10.1-20121218-UNOFFICIAL-tf300t
ro.modversion=10.1-20121218-UNOFFICIAL-tf300t
ro.config.ringtone=Orion.ogg
ro.config.notification_sound=Deneb.ogg
ro.config.alarm_alert=Hassium.ogg
dalvik.vm.lockprof.threshold=500
dalvik.vm.dexopt-flags=m=y
net.bt.name=Android
net.change=net.bt.name
dalvik.vm.stack-trace-file=/data/anr/traces.txt
init.svc.ueventd=running
init.svc.recovery=running
init.svc.mischelp=restarting
init.svc.adbd=running
service.adb.root=1

I:Checking for extendedcommand...
I:Skipping execution of extendedcommand, file not found...
SD Card space free: 30424MB
Backing up boot image...
Incorrect mbr signatures!
Error in reading mbr!
Error while backing up boot image!I:using /data/media for /emmc/clockworkmod/backup/2013-01-11.00.25.47.
SD Card space free: 16970MB
Backing up boot image...
Incorrect mbr signatures!
Error in reading mbr!
Error while backing up boot image!Formatting /sdcard...
I:Formatting unknown device.
rm: can't remove '.' or '..'
rm: can't remove '.' or '..'
Done.
SD Card space free: 30424MB
Backing up boot image...
Incorrect mbr signatures!
Error in reading mbr!
Error while backing up boot image!I:using /data/media for /emmc/clockworkmod/backup/2013-01-11.00.36.36.
SD Card space free: 16970MB
Backing up boot image...
Incorrect mbr signatures!
Error in reading mbr!
Error while backing up boot image!I:Can't format unknown volume: /external_sd
I:Can't partition non-vfat: datamedia