ReadyNAS RNDU6000 memory upgrade - won't boot

That's the right package, but it won't work with OS6. But before you do anything else, what version does your bios_version.log have in it?  You may not need to update.

If you do need to update, there are instructions elsewhere in the forum (I don't have a link) about how to extract and install the BIOS under OS6, but the easiest way for most is to use a spare drive in your NAS instead of the normnal volume drives and revert back to OS4.2.x, then install the BIOS update add-on, then revert back to OS6 before re-inserting your normal drives.

/sys/class/dmi/id/bios_date
::::::::::::::
05/26/2010
::::::::::::::
/sys/class/dmi/id/bios_version
::::::::::::::
080016

Is this instruction still correct?

Definately an older version.  I can't say if the update will fix the issue or not, but it's always best to have the latest if you can.

So I just followed the instructions I found. Little change for Ultra6.

This are the commands I succesfully used (for reference):

# install tools
apt-get update
apt-get install dmidecode flashrom
apt-get install libc6-i386
# download bios package
cd /tmp/
wget http://www.readynas.com/download/addons/x86/4.2/BIOS_Update_Package_0.5-x86.bin
# unpack bios package
dd if=BIOS_Update_Package_0.5-x86.bin bs=16k count=1
dd if=BIOS_Update_Package_0.5-x86.bin bs=16k skip=1 | md5sum -
dd if=BIOS_Update_Package_0.5-x86.bin bs=16k skip=1 | tar -xv
# this unpacks following files in /tmp
#    install.sh
#   remove.sh
#   addons.conf
#   files.tgz
# now unpack files.tgz
tar -xvf files.tgz
# this unpacks following files
#    ./opt/biosflash/flashrom
#    ./opt/biosflash/RN_3200_062410.ROM
#    ./opt/biosflash/RN_Ultra6_061010.ROM
#    ./opt/biosflash/RN_NV6_072610.ROM
#    ./opt/biosflash/RN_Ultra4_082311.ROM
#    ./opt/biosflash/RN_Ultra2_110211.ROM
#    ./usr/sbin/dmidecode
# it actually unpacks a flashrom utility and dmicode...
# using install.sh as reference for the correct -c (chip) for my Ultra6, I tried:
flashrom --programmer internal -c W25Q80 -w /tmp/opt/biosflash/RN_Ultra6_061010.ROM
# got an error: Error: Unknown chip 'W25Q80' specified.
# now tried again with the flashrom that came with the package
/tmp/opt/biosflash/flashrom --programmer internal -c W25Q80 -w /tmp/opt/biosflash/RN_Ultra6_061010.ROM
# within seconds: Verifying flash... VERIFIED.

Checked the bios_ver.log log:

::::::::::::::
/sys/class/dmi/id/bios_date
::::::::::::::
06/10/2010
::::::::::::::
/sys/class/dmi/id/bios_version
::::::::::::::
080016 

So all went well!

Sadly 4GB still not recognised. So still running on 3GB.

You possibly might have had to user lower case for the chip name. I think with a few models there might be some systems that used a different chip. Can't remember whether the Ultra 4 was one of those.

Hmm, sounds plausable but seeing that the update was already succesfull with the packaged tool i'm not gonna test that out :)  Not going to touch that ever again!

Indeed. BIOS updates are not without risk. If for example the power failed during a BIOS update a device could be bricked.

Was just pointing out in case others came across this thread later.