Thanks DerikC :) After experiencing this problem on another Dell system with a different Seagate update it appears to be related to systems with Exchange installed on SBS (Small Business Server) systems. Taking your I/O comments on board I shut down all services (including Exchange and SQL) and verified from performance monitors that the disks were idle. However the update still failed (on both systems). I then scheduled the update for a regular patch round, and applied immediately after reboot. It then worked fine on both systems. Strange given that the I/O on startup was very high as services are starting and the Exchange store is mounting, but the update still completes OK. Therefore to assist anyone experiencing this issue, it is definitely recommended to take the advice of applying immediately after a restart of the server (regardless of I/O). The invalid image file error message is misleading, as it implies there is something wrong with the update itself, corruption or otherwise. There does appear to be a correlation with the length of time the server has been running and the failure of the update, regardless of I/O, which a reboot fixes. Anyway, problem now solved! Hope this helps other people in future.
Attempting to update a T420 with 2xST2000NM0001 using this package: www.dell.com/.../DriversDetails;fileId=3325752923&osCode=WS8R2&productCode=poweredge-t420&languageCode=EN&categoryId=AS Filename SAS-Drive_Firmware_GXCDR_WN32_PS08_A04 Error given is that the update failed due to an invalid image file. I can see from the log file that this update has been attempted several times since it was released so the problem has been there since release. Last part of log file: ===============> Update Package application started <===============Command:    C:\installs\SAS-Drive_Firmware_GXCDR_WN32_PS08_A04.EXE Date:        2015-05-27 15:19:37=========================================================================CheckMSIPMIDriver : dwStartupType :3Release ID: GXCDRA00-00Update Package version: 7.3.1 (BLD_77)User: redacted\redactedCollecting inventory...Running validation...The version of this Update Package is newer than the currently installed version.    Software application name: ST2000NM0001 Firmware   Package version: PS08   Installed version: PS07The version of this Update Package is newer than the currently installed version.    Software application name: ST2000NM0001 Firmware   Package version: PS08   Installed version: PS07Executing update...Device: ST2000NM0001, Application: ST2000NM0001 Firmware   The operation failed due to an invalid image file.Device: ST2000NM0001, Application: ST2000NM0001 Firmware   The operation failed due to an invalid image file.==============================> Update Result <==========================Update was not applied=========================================================================Exit code = 1 (Failure)2015-05-27 15:21:25===============> Update Package application started <===============Command:    C:\installs\SAS-Drive_Firmware_GXCDR_WN32_PS08_A04.EXEDate:        2015-05-27 15:21:25=========================================================================Update Package finished.  Exit code = 12015-05-27 15:21:28
Hard disk package update failing