So is it that a single partition drive (NTFS?) won't boot (to what and what format was it in originally?) and that a 2-partition drive would boot?
I think you need to try some more tests...
e.g. re-format as single NTFS ptn and try, re-format as NTFS+small ptn (in RMPrepUSB 'Boot as HDD') and try again.
These were 3 Passport 1TB USB 3 drives, all formatted with a single NTFS volume of just under 1TB in size , previous to the Dell E7240 laptop that I had boot issues with yesterday it booted on damn near anything I put it on. I am using your Easy2Boot and a scad of ISOs. I mainly booted to Windows 7PE and applied a ghost image to new PCs.
Putting it on this laptop and pecking F12 and choosing USB produced an INVALID PARTITION error first thing before it even tried to boot anything, it just ground to a halt and threw this error. I whipped out my trusty USB 3 JumpDrive128GB NTFS formatted thumb drive and it booted fine, so the issue was not NTFS nor seems to be USB 3. I have a few 500GB NTFS formatted USB 2 PassPort drives and they boot just fine. As did any lower capacity drives I tried (all with single NTFS primary boot partitions the full size of the drive). I also had an older 1TB NTFS USB 3 Passport drive that is maybe 1 year old. When I first set it up I had issues with it booting but it simply was a Grub4DOS issue, for some odd reason it would not find the grldr file and would bomb out. Way back when I messed with it for a few days and then gave up and set up a FAT32 formatted primary partition with an NTFS second partition, reinstalled Grb4DOS and copied the files to it, it booted fine from then on. This is what made me thing of the single NTFS primary partition being the issue.
One of the newer 1TB drives had just a small amount of data on it, so I booted to Parted Magic and shrank the 1st primary partition to 50GB, made sure the boot flag was enabled and tried it on the screwy laptop. it booted fine. So I created a second NTFS partition in Windows and tried it again. And again success. I juggled the data on the other drives, shrank the first NTFS partition and created the second, all now boot fine.
So I am not sure if it's this funky laptops USB controller or what. Yesterday I also had a few Dell E5430 laptops which booted just fine with the single large NTFS partition scheme, as well as the new smaller scheme.