Why does Android drop files I transfer to SDCard while in recovery mode? Here's my situation:

I have a Sprint Samsung Nexus S 4g.  It is rooted, unlocked, and loaded with CyanogenMod 10.1.
I am attaching it to a Windows 8 machine via USB cable.  

Recently, the phone went into a boot loop, and thus I can only do anything with it in recovery mode.

I'm trying to transfer the stock ROM to the phone for loading.  I follow these steps


I boot into Recovery mode
I go to Mounts and Storage
I mount the USB card
In Windows, I open up the phone in explorer, and copy the update.zip file to the root of what is now the G: Drive on my phone.  
I  try to load the ZIP file for processing.  Recovery says "Can't open file /sdcard/update.zip"  
I reboot into recovery again to 'refresh' things.


At this point, the file that I loaded is gone!  It's not on the SD Card anymore.  I can't find it via Recovery mode, and if I remount the USB disk, it's not visible in Windows either.  Weird, huh?

One other thing to note:  I've wiped and reformatted everything I can via Recovery (Cache, system, data, Dalvik, everything) and yet when the phone occasionally manages to boot all the way to the OS, I can still see my wall paper. 

How can I get the files I copy over to actually stay on the phone?  Any ideas on what is going on?

I just want to wipe this phone and start over, but I can't seem to make that happen.  

Thanks.

More:

Okay, I formatted and wiped everything that I could.  And now the phone only boots to fastboot mode.  I mount the USB card, move a ZIP over, and double check it in Windows.  I can open the file on the phone.  But when I unmount the USB card (I have no choice), the ZIP is there, but the phone won't read it.  It says "Can't open  (bad)".  This is trying with CM9.1 and I'm 100% sure it is the right version for my phone (crespo4G)
Try downloading the update.zip again to ensure it's not a bad download.
Try updating your version of ClockworkMod Recovery, i think the latest version for your phone is 6.0.3.1. Get into fastboot mode and ensure you have the drivers installed. You can flash this by downloading the relevant file and running the command
fastboot flash recovery &lt;recoveryfile&gt;
(replace  with the filename of the recovery you downloaded (eg recovery-clockwork-touch-6.0.3.1-crespo4g.img)


If that fails you could try flashing the factory image again. Download the factory image from here and unzip. There should be a flash-all.bat which will restore your phone to stock. Note that this operation deletes all user data by default. (although i guess you've already wiped so nothing left to delete!)