[HD2 Windows Mobile 6.5 Q&A, Help & Troubleshooting] Snarksneeze:
I've been asked this a few times and I actually had to get help the first time I tried it, so here's my tutorial on how to flash a ROM or Radio image using your microSD Card. I personally use this method because my own computer runs Linux. I could probably boot up to Windows from time to time, but that would be like cheating on my wife; I just can't bring myself to do it if I can avoid it at all ;)

A few of us old-timers to HTC are so used to our personal little tricks we use that we forget that newer users haven't learned them yet. I'm expecting a few people to say, "Oh yeah, we already knew that!" but perhaps one or two will instead learn from it and appreciate it.

So, without further ado:

Different chefs use different methods when naming their ROM or Radio images. You must rename the image to "LEOIMG.nbh", since this is the only recognized file name the bootloader will accept. (note: Some people are reporting that the file name must be capitalized, or it will not work. When named exactly as above, it has always worked for me!)

Place the LEOIMG.nbh on a 8GB or smaller microSD card.

Remove the battery cover and press the Reset button.

Hold the Volume Down and End Call (power) buttons at the same time.

As soon as "Searching..." appears on the screen, release the Volume Down button.

ROM will flash. Be sure to hard-reset afterwards to prevent weird errors (as usual).

Now, in case you are wondering about the specifics:

Remember to put the file on the "root" of the device. This means you can't put it in a folder. I am using an 8GB Class 2 card for flashing. I would recommend using a 1GB Class 6, but it's not necessary. It may be possible with a higher class 16GB card, but I haven't tried. Our Class 2 that came with the phone never works, though I don't rightly know why. [update]souljaboy wrote in and said: "it doesn't work because a fat32 is needed and by default the card is formatted fat, I think you should mention that, it worked for me!" Thanks, souljaboy!

The reset button is a small red or yellow spot just below the slot for the microSD card. You could also remove and replace the battery; the important thing is to shut your phone off and restart it.

Pressing Volume Down and End Call (or Power) is the recommended way to enter the bootloader. If you press the Volume Up and Volume Down at the same time, you will enter the "Hard Reset" option instead.

If "Searching..." does not appear, you either do not have a compatible card in the phone, or you are not holding the right buttons. If you hold the "Volume Down" button past this point, you will see a message that says, "Press the Power Button to upgrade your phone." Releasing the "Volume Down" button at this point may or may not work, I had to release it before. If you release the "Power Button" now, your phone will simply reboot without flashing.

ROM flashing via SD is much safer than via USB, since you don't have to worry about accidentally unplugging the cord or an issue with your PC, such as a crash or power failure.

---Added 05/21/10 as suggested by zelendel (Thanks for the sticky!)

If you are flashing an "Official" ROM direct from HTC or T-Mobile, you will more than likely over-write your SPL (Second Program Loader). This means, even if you have HSPL installed now, you could be reverted back to SSPL, rendering you unable to flash custom ROMs.

To make matters worse, if you flash an ROM higher than 2.10 and you attempt to install the current (as of 5/21/10) HSPL, you will brick your phone. Yeah, it's bad.

So, assuming you have flashed an official ROM over 2.10 via the SD card and you decide you want a custom ROM, follow these simple directions:

(You do not have to have your phone connected to your PC for these steps)

Download the "Official" ROM from here. (Thanks to zelendel for the upload!)

Follow the instructions above to flash your phone with this image.

Once you phone has fully booted, I recommend doing an extra hard-reset.

Next, visit mskip's thread to download the latest HSPL and flash that (you can flash it either via RUU or via microSD).

If done properly you should now be able to once again flash custom ROMs. Congratulations!

Hopefully someone will find this useful. If you have any issues, reply to this thread and I or some other wonderful xda member will try and sort things out.

Happy Flashing!