After hooking up your device to the PC you have to manually choose to "mount" the usb drive on your MyTouch. It's in the notification bar. I read this in the user's manual and it worked for me:
https://support.t-mobile.com/doc/tm30284.pdf?&A2L.SERVICE=UserManual
1.13 Using the Phone’s Storage Card as a USB Drive
To transfer music, pictures, and other files from your computer to your phone’s storage card, you need
to set the phone’s storage card as a USB drive.
To set the phone’s storage card as a USB drive
1 Connect the phone to the computer using the USB cable. A notification icon appears in the status
bar.
2 Open the Notifications Panel. Refer to “To open the Notifications panel” on page 27 to learn how to
open the Notifications Panel.
3 In the Notifications Panel, touch USB connected and then touch Mount.
When you select Mount, the phone will not recognize the storage card when it is connected to a
computer. You will not be able to use some of the phone’s applications such as Camera. You need to
turn off USB storage for the phone to recognize the storage card when the phone is connected to a
computer. Refer to the next procedure to learn how to turn off USB storage.
To turn off USB storage
1 Open the Notifications Panel. Refer to “To open the Notifications panel” on page 27 to learn how to
open the Notifications Panel.
2 In the Notifications Panel, touch Turn off USB storage and then touch Turn Off.
To safely remove the storage card while the phone is on
When you need to remove the storage card while the phone is on, you must unmount the storage
card first to prevent corrupting or damaging the storage card.
1 Press HOME, then press MENU, and then touch Settings.
2 Scroll down the screen, then touch SD card & phone storage.
3 Touch Unmount SD card.
Remove the storage card by following the instructions in “To remove the storage card” on page 18.
To format the SD card
1 Press HOME, then press MENU, and then touch Settings.
2 Scroll down the screen, then touch SD card & phone storage.
3 Touch Unmount SD card. This enables the Format SD card option.
4 Touch Format SD card > Format SD card.