Your assumption of higher efficiency is not necessarily correct. High energy efficiency is a general feature of the switched-mode power supplys of which type the stock Drobo “brick” is as well. Of couse there could be some few-percent differences, maybe in favour of your server’s power supply but since you know only the figure for one of them, you could only speculate and generally the difference won’t be big enough to justify your idea from the economic point of view. Idea which I don’t like at all, to be honest. You’re going to put some serious strain on one particular 12V rail of your computer’s PSU. My Drobo came with 80W power supply which means that, at least in theory (if the designers didn’t leave much “headroom”) you could be draining as much as ~7 amps of current from a single 12V rail of your PSU with the Drobo alone. A rail which is probably hooked into something already, anyway. Furthermore, by wiring it your way, you’re sacrificing Drobo’s “self-containeness” and, in my opinion, you make the things much more messy and cumbersome than by just making an adapter cable from the UPS to the Drobo stock PSU directly. Your PC’s PSU would also became a single point of failure (which is more probable given the much higher stress put on it).
Don’t go this way. If you were going to implement your idea, that means you’re already familiar with a soldering iron because I don’t see another way to make a ‘PC PSU->Drobo supply jack’ adapter. If so, just buy one piece of UPS->PC cable, one piece of “clover leaf” power cable for Drobo PSU, cut them in half, solder them together (mind the wire’s colors!), insulate them carefully and you’re good to go. Both PSUs will stay nice and cool and hopefully will live happily ever after.
Oh, one more thing. When connected your way, the Drobo would turn on and off in sync with your PC. The 12V rail in PC PSU goes up and down with the “main” PC power which means that Drobo would go completely down as soon as you shut down your OS or put it to standby. Given so, it would go through its lengthy dancing-lights-power-up-and-initialize procedure each time the PC goes up/wakes from standby which means that almost certainly it wouldn’t be ready by the time OS is already up and running. Drobo is clearly not designed to be power-cycled this way. Furthermore, there’s one more concern of the possibly high initial current “spike” at power-up which could harm both your PC PSU or UPS. There is some more voltage conversion going on further inside of the Drobo (it has to make 5V for the drives and even less (3.3V possibly) for its internal logic) and if these DC-DC converters work in switched-mode also, there is a fair chance for such “current-spike” to occur while powering the device on.[hr]
There it is:
“Efficiency: 82% - 87% typical”
You can also look for a 80S12-V model which already has the UPS-type socket. Or similar 12V, 80W+, center-positive switched PSU.