Hello,
Couple of questions for you on your configuration from the host side. Do you have MPIO installed correctly? Do you have two paths to the enclosures? Straight thru or redudant cabling? Have you unmapped the LUN/volume on the D2600 you are trying to decommission?
From your description it sounds like you have a software host side issue where the LUNs are interconnected. Did you use the SSA to configure the LUNs or a host based software solution?
However, there is not enough information here to determine your current configuration with all three enclosures connected. You mention there are 4 LUNs yet three enclosures. This inidcates you have at least one enclosure that either has 2 LUNs or you have one LUN that exists across two enclosures. With SATA HDDs in the first two enclosures are you sure you do not have one LUN spanning those two enclosures?
I would urge a backup of your important data before proceeding further. One consideration is to perform the backup delete all the LUNs, remove the enclosure and then you can recreate the LUNs according to your new plans. I understand that is a lot of work but would leave you with a fresh install and remove any issues.
Cheers,
Shawn