If your pc doesn't recognize your phone in fastboot make sure your sdk is up to date including thirdparty installs, and download pdanet trial version and those drivers will work. get drivers here http://hotfile.com/dl/92897696/7ae5290/drivers.rar.html
or if your not comfortable installing drivers download pdanet here and install as it will automatically install drivers for you
Ok download the files attached and extract them. For the first part for rooting you will need the:
-Ok put the superuser.zip on the sdcard of your phone.
-Power off phone, when off hold volume up and power button to get to bootloader.
-Go the the tools folder of your android sdk and open command prompt and enter the following command: (easiest way is to hold shift and right click on tools folder and hit the button that says "open command window here" then type these command...
fastboot devices (make sure your device shows up, if not your drivers aren't installed)
fastboot oem unlock
-this will unlock the bootloader and you should follow the prompt and accept (volume up, then power button)
-Some have reported unlocking the bootloader erases sdcard..if so when you reboot after unlocking make sure to put superuser.zip back on sdcard.
-this should reboot your phone, now power off, when off hold volume up and then power button to get back to bootloader.
-place the recovery3101.img in the tools folder and type the following command
fastboot flash recovery recovery3101.img
-You should see it say flash ok in the command prompt, now scroll down (on phone) to recovery and hit the power button (ok button)
-Go to flash zip from sdcard and select the superuser.zip and let it flash.
-when done reboot phone, sign into google accnt, download rom manager, open up rom manager go down to All clockworkmod recoverys and flash 3.1.0 (the newest recovery doesn't work for me as my phone can't get into recovery that is why we are flashing the other one )
-All set you can flash other roms, MAKE SURE TO CREATE BACKUP OF YOUR CURRENT STOCK ROOTED ROM FIRST.
Get files http://www.4shared.com/file/N5g8Hav_/root.html?]
-download and extract files, we will be using odin, the 3 tar files.
-extract tar files to desktop along with odin.
-run odin and in the files section select the boxes bootloader, pda, phone.
-click the buttons next to each and direct it to the .tar files we extracted to desktop, so....
click the bootloader box, direct it to sph-d720-bootloader.tar ...then..
click the pda button direct it toward the sph-d720-pda.tar....then..
click the phone button and direct it toward sph-d720-phone.tar....
-now turn off your phone, when off hold volume up and volume down buttons simultaneously, then quickly press the power button....a huge yellow triangle will pop up and say "downloading'.......
plug your phone via usb to computer....odin should automatically see it.....click the start button in odin and let it do its thing and bam...all set.
If you want stock recovery too place the recovery.img in tools folder, open command prompt..type the following command..
fastboot flash recovery recovery.img