Anyone taken a C6100 with 24 bays and had all drives go to a single node?
I have c6145s and c6100s. They are very similar inside, and share quite a few parts, including, I suspect, the drive backplanes. That provides you with an opportunity.
If you look on back side of the 24-drive backplane of the c6100, you should see something that looks like a PCIe connector. In the c6145, this port connects to a custom 36-port expander board with an LSI chip, part number 71N1C. If you install the c6145 expander board into your c6100, you may get what you want. I say "may" for two reasons: 1) I haven't tried it on the c6100 myself 2) There are two firmware versions for these expanders. One (shipped with single-node C6145 servers) wires all 24 drives to two SFF-8087 ports, and the other (shipped with 2-node c6145s) wires 12 drives to one SFF-8087 port and the other 12 drives to two other SFF ports. If you get the wrong version, Dell support should be able to help you convert it, since it's just a configuration.
Check eBay for these expanders - there was one available last week. Here's a photo:
In the photo, the six leftmost SFF-8087 ports (which look odd because they have blanking plugs in them) wire to the drive backplane using very short breakout cables. The three rightmost SFF-8087 ports wire to the c6145 midplanes, into which the interposers insert themselves. In a single-node c6145, only two are used. In the dual-node c6145, all three are used, with two connecting to one node and one connecting to the other. Yes, that gives one node more bandwidth than the other. This was unavoidable since LSI didn't make 40 port expanders. By the way, the PCI-looking connector is, I believe, used only to provide power to the expander board.
Alternatively, if you like the expander concept but prefer hacked solutions with cables running everywhere, you could see if there was room for an Intel RES2SV240 expander in the drive area forward of the fans - it might fit in a 24-bay with it's shallower drives. If it fits, wire 20 drives to the expander with SFF-8087 to SATA breakout cables and then wire the sixth SFF08087 connector to the interposer board - making sure that you use the "four drive" connector on the interposer and not the "two drive" connector. You can wire two of the remaining drives to the interposer using the existing cabling, leaving just two drives not used, or wired to other nodes. In this solution, you'll need to find a Molex cable to power the expander.