It asks for it, but I don't think it does anything with it when you only have the RPG and COBOL tools installed.  I added 
https://www.ibm.com/software/repositorymanager/com.ibm.rational.developer.ibmi.v90 as a repository in Installation Manager and saved having to download it.  Didn't take appreciably longer for the update, either - of course, I have a reasonably fast connection.  I think it's a good alternative.
 
Dave Shaw
________________________________
 From: Sam_L <lennon_s_j@xxxxxxxxxxx>
To: wdsci-l@xxxxxxxxxxxx 
Sent: Thursday, September 26, 2013 8:35 PM
Subject: [WDSCI-L] RDP 9.0.0.1 and Disk2
 
I downloaded RDP 9 from the ESS Site.  (Lots of stuff, about 13 GB...)
I unpacked the "RDi_Core_Product_1_V9.0_LCD8_1991-00.iso" file and 
installed.  I installed just the RPG and COBOL tools.
Now I'm trying to install 9.0.0.1.  Installation Manager downloaded some 
stuff, but now it is asking for Disk2.  What?
I unpacked ""RDi_Core_Product_2_V9.0_LCD8_1991-00.iso" to a new 
directory and pointed it a that.  And off it went and upgraded to 
9.0.0.1.  Yah!  But I'n not sure why it needed Disk2.
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