Just make sure you're following the correct procedure. You do not need to root the phone to install CM7 from stock. If you're not exactly sure how that goes it's pretty simple.
Pull battery, put phone into download mode
ODIN the fixed CWM4 JT has posted in several threads
Immediately boot up the phone using the 3-finger method, you can only do this once. After successfully installing an AOSP rom you can't use the 3-finger method anymore.
Wipe the big 3, then install the rom.
If you're having trouble getting the SD card to mount, you may want to try formatting it or at very least try another card. It could be damaged from using a previous nightly when there were known issues with corrupted SD cards.
Pull battery, put phone into download mode
ODIN the fixed CWM4 JT has posted in several threads
Immediately boot up the phone using the 3-finger method, you can only do this once. After successfully installing an AOSP rom you can't use the 3-finger method anymore.
Wipe the big 3, then install the rom.
If you're having trouble getting the SD card to mount, you may want to try formatting it or at very least try another card. It could be damaged from using a previous nightly when there were known issues with corrupted SD cards.