Ugh, we have some 270s at work still lol. I do not like them.
This connector is an expansion of a simple 3-pin female IC connector by adding two tabs to the middle of the connector on one side and a lock-tab on the other side. The size and spacing of the pin sockets is identical to a standard 3-pin female IC connector and 3-pin Molex connector. Some models have the wiring of the white wire (speed sensor) in the middle, whereas the standard 3-pin Molex requires the white wire as pin #3, thus compatibility issues may exist.
Wiki on "Computer fan" indicates Dell uses a proprietary connector. If it's looking for some sort of reading from it before boot, that may be the reason for it. I looked at one of our 270s in the back and it's using a three pin connector at least.
Also you might be able to simply go into the BIOS and disable those protections.