I could not find any way to issue a "no slot" command but I actually got this problem solved by pretty much doing what you suggested. The strange part of the problem is that the SFP+ cards worked without any setup issues in both slots, as did the 10GBaseT card in the second slot. No manual configuration is needed in all the cases above. The solution to my problem with a 10GBaseT card in the first slot was solved in exactly the same way on both swtiches (7048P).  1. Unplug the card in slot 1. 2. In Openmanage go to "Switching > Slots > Card Configuration 3. Select slot 1 and DELETE the 10GBaseT setting that was automatically picked up when the card is inserted. 4. Re-add the 10GBaseT card for slot 1. 5. Plug in the 10GBaseT card in slot 1. (in this step I copied the running config to the starting config but as far as I can tell that shouldn't be necessary as slot setup is not in the config file) 6. Reset/reboot the switch. This was a weird one as the second slot is 100% plug and play.  Thanks for leading me in the right direction Joey.
We have two Powerconnect 7048P running at separate locations and both switched works just fine with 10GBE SPF+ modules populating both expansion slots. All four (2x2) ports link up. We recently swapped both modules in one of the switches to 10gbase-t 0TNTFF (RJ45) modules. Both modules show up as expected in the GUI and via console but only the ports on the second slot module actually links up. I first thought the switch may be malfunctioning so I installed the RJ45 modules in the other switch but exactly the same issue occurred. Swapping the expansion modules around makes no difference, still only the second slot links up. No lights at all on the first slot. In the documentation for the 70XX switches I cannot find any information explaining my issue. In the documentation for the 60XX series it is clearly noted that the RJ45 module can only be installed in one of the two expansion slots. Is this a matter of bad documentation or (as I'm hoping) a matter of bad configuration? The second slot "just works" as do the SFP+ modules. 
Powerconnect 7048P with dual 10gbase-t modules (0TNTFF) only links up on second 
slot