It might be a bad SD card... do you have another SD card you can try?
Also try downloading the 4431 file again, maybe it was corrupt.
You said you have 3 of these?
did you try a firmware update on all 3? if not try the other ones. Maybe 1 in 3 is bad?
If anyone is wondering... the instructions from LePan are:
Le Pan Tablet PC TC970 (Wi-Fi) - Upgrade Process:
STEP 1: Please remove the Micro SD Card from device, use card reader to read and then delete the card contents on PC.
STEP 2: Copy the download Apps and the new software file to root of Micro SD Card.
STEP 3: Insert the Micro SD Card into Tablet.
STEP 4: Press and hold [Vol-] and [Power] key to turn on your Tablet PC, once logo shows up, release [power] first and then [Vol-].
STEP 5: Press [Vol-] to move the light bar to select “Yes——Install the firmware”, then press [power] key to start the upgrading. [Note: if battery is lower than 60%, please use wall adaptor to charge the Tablet.]
STEP 6: Firmware installation in processing:
STEP 7: With [Vol] key to move the light bar to Reboot system now and with [power] to confirm. Then Tablet will restart.
STEP 8: Once device is on, please follow the direction to set up the time etc.
STEP 9: Press Menu or Applications , and then touch File Manager, install below 5 APK docs: - Adobe Reader - Adobe Flash Player - GoogleBooks - iReader - Kindle
STEP 10. After installation of Apps, upgrade finish.
----- THE END ----
Also, you might want to try these threads over in XDA, they talk about rooting but if you read through, they mention the firmware update as well. Maybe someone over they can help.
Le Pan TC970 rooting problems help? - xda-developers
Root Achieved for Le Pan TC 970 - xda-developers