Thanks for the post. I would first suggest checking in the BIOS currently installed on your system if the Signed Firware update (opt in) in the BIOS can be found under the security settings and is listed as "Signed Firmware Update". Once this feature is turned on, it cannot be disabled.
If this setting is enabled the system will checks all BIOS updates are dell signed prior to completing installation, If for some reason the .exe file is not signed or corrupted, the installation will fail.
If the setting is disabled. Have you been able to try reinstalling A07 or even flashing back to a previous version before upgrading to A08?
If the setting is enabled. I would suggest downloading the A08 file again to make sure the version already downloaded was not corrupted in its original download.
For more information on the signed driver settings and BIOS updates see below:
1. Please note that if the A01 BIOS is currently installed on your system you must
first apply the P02 patch (M4500P02.EXE) before upgrading to this BIOS.
P02 BIOS link: http://www.dell.com/support/drivers/us/en/04/DriverDetails?driverId=R271142
2. Please note that if the A07 or before A07 BIOS is currently installed on your system
you must first update to A08 BIOS and then flash to the latest A-rev BIOS.
A08 BIOS link: http://www.dell.com/support/drivers/us/en/04/DriverDetails?driverId=J2M4W
3. Please note that if the A09 or before A09 BIOS is currently installed on your system
you must first update to A10 BIOS and then flash to the latest A-rev BIOS.
A10 BIOS link: http://www.dell.com/support/drivers/us/en/04/DriverDetails?driverId=R3GXK
Install in the order below. Make sure to hit <F12> and go into BIOS setup after each one and make sure that the Only Allow Signed BIOS is NOT CHECKED. It's in the security section of the BIOS.
M4500P02 Search for M4500A02, but you will get M4500P02, which is actually A01.1
I am on my M4500 typing this right now and had the EXACT same situation you did. Bought a refurbished M4500 with A07 on it.
Hope this helps!