First of all, holy shit this was a miserable experience. The tl;dr version is that on a 2008 Mac Pro (2 x 3.2GHz Quad-core) running a freshly installed OS X 10.6.6 with rEFIt 0.14 I couldn’t successfully install a Windows 7 x64 ISO from USB.

To successfully install Windows 7, I ended up driving out to buy some DVD-Rs and burning the ISO in a Win7 VM (VMWare Fusion 3.1.12) installed from the ISO via Boot Camp. If you are trying to install on a machine without a DVD drive, I suggest either simply using a VM or if you must Boot Camp, acquiring an external drive.

Additional caveats: once I was able to boot was that the Windows 7 installed did not recognize the BOOTCAMP NTFS partition, and the after formatting, threw an apparently common Error 0x80300024 on install (the solution for this was to shut down and pull out my 3 other drives out so the Windows installer didn’t get confused).

At the end of the ordeal, I ended up with a working Windows 7 install that used 28.4GB of space. o_O

Disabling components actually only increased usage. There is a tool called DISM (must be run as admin) that can manage packages, but appears to treat almost the entire Windows 7 install as one big package. The only way to strip Windows 7 down seems to be using a tool like RT Seven Lite to create an ISO that removes the packages entirely beforehand.

Note: if you need to zap the BOOTCAMP volume during the Windows 7 install, you’ll need to grab the Boot Camp drivers from the Snow Leopard CD or from BitTorrent (Apple doesn’t offer 3.0 drivers standalone, and 3.1 and 3.2 require 3.0 to be installed) and run those manually. (You may need to do this anyway as apparently Boot Camp 3.0 may fail by trying to installing the x86 instead of the x64 drivers).

In case you didn’t get it by now, this experience has been a pretty convincing argument for continuing to stay away from Windows as much as possible, certainly on Macs where the unholiness combination of EFI, Boot Camp, and Windows just makes a complete mess.

For your edification/amusement/horror, additional things that did not work on my Mac when trying to install from USB:

dd

The recommendations to use dd if=$iso of=/dev/$usbkey -bs=1m is a total fail as it’ll create a UDF image on the USB key, which apparently the Mac’s EFI won’t detect

Windows 7 USB/DVD Download Tool

I had an older Windows XP partition, so I was able to try Microsoft’s ISO tool. This created an image that was bootable in the sense that rEFIt could see and select it, but subsequently did not show up as a boot device (DOS prompt for inserting a bootable disk)

UNetbootin

Another try once I started down the WinXP path, I tried using UNetbootin to make the USB stick bootable, but to no more success. Same result as the Microsoft ISO tool.

WinToFlash

Next into my increasingly desperate descent into madness was to use an app that could write Vista/7 boot procedures using EFI. Unfortunately, these resulted in an “Error: Unsupported while loading bootmgr.efi” message when booting in rEFIt

Windows 7 in VMWare Fusion + DISKPART

Finally, I decided to install the ISO in VMWare Fusion on OS X, and then follow this method with DISKPART (you can’t do it in XP because it’s version of DISKPART doesn’t recognize removeable drives as disks). This also failed spectacularly (also, trying bootsect), at which point I threw in the towel on USB booting.

UPDATE: Here’s a method of getting BootCamp Assistant to create a bootable USB Windows installer that might work, although I haven’t tested it at all and it’s probably also dependent on how old your Mac is if it does work…

UPDATE 2: On a recent PC, I was able to make a bootable install disk via Linux which may be helpful, but may not be applicable w/ old Macs w/ 32-bit UEFI.