I have never downgraded my wd mbl firmware
not sure if is even a good idea in some scenarios
but the question did come up in another forum
It got me to thinking, about where and how one would do that
WD does not make it easy to get links to the older firmware
not sure if these are also applicable to the wd mbl duo
By reading the current release notes for apnc-021109-053
I was able to do some searching to find links to all the known firmware files
http://support.wdc.com/download/notes/My_Book_Live_Release_Notes_FW02_41_05.pdf
http://support.wdc.com/download/notes/My_Book_Live_Release_Notes_FW02_32_05-046.pdf
http://support.wdc.com/download/notes/My_Book_Live_Release_Notes_FW_Patch_02_32_05-144
http://support.wdc.com/download/notes/My_Book_Live_Release_Notes_FW02.11.09-053.pdf
Here are the links that I found for 2.x firmware files
http://download.wdc.com/nas/apnc-024303-022-20130916.deb
http://download.wdc.com/nas/apnc-024203-027-20130529.deb
http://download.wdc.com/nas/apnc-024202-012-20130401.deb
http://download.wdc.com/nas/apnc-024105-034-20130215.deb
02.40.00-017
02.32.06-006
http://download.wdc.com/nas/apnc-023205-046-20120910.deb
http://download.wdc.com/nas/apnc-023205-044-20120823.deb
02.11.12-060
http://download.wdc.com/nas/apnc-021109-053-20120413.deb
http://download.wdc.com/nas/apnc-021012-129-20120308.deb
http://download.wdc.com/nas/apnc-021009-124-20111113.deb
http://download.wdc.com/nas/apnc-020301-024-20111019.deb
http://download.wdc.com/nas/apnc-020202-020-20110825.deb
http://download.wdc.com/nas/apnc-020106-026-20110801.deb
http://download.wdc.com/nas/apnc-020035-423-20110722.deb
Here are the links that I found for 1.x firmware files
http://download.wdc.com/nas/apnc-010507-20110714.deb
http://download.wdc.com/nas/apnc-010406-20110407.deb
http://download.wdc.com/nas/apnc-010303-20110119.deb
http://download.wdc.com/nas/apnc-010203-20101018.deb
http://download.wdc.com/nas/apnc-010103-20100930.deb
These are for the wd mbl duo
http://download.wdc.com/nas/ap2nc-024006-048-20121206.zip