Seagate's HDD firmware is divided into functional modules. Some modules reside in the "ROM" chip on the PCB, while the remainder reside is a hidden System Area (SA) on the platters.
Updating Seagate OEM firmware (eg Hewlett Packard HP35) with retail firmware (CC49) has serious risks. This is because a firware update does not typically replace all firmware modules. This means that the resulting firmware will be a hybrid mix of OEM and retail modules. These may or may not be compatible. The result may be a non-functional drive, or a drive that behaves strangely.
For example, Seagate specifically states that you must not apply a CCxx firmware update to a drive with SDxx firmware, and vice versa. These are both retail versions. If you ignore Seagate's warning, the result will be a bricked drive.
Here is a case where a user applied a retail update to OEM firmware:
https://forum.hddguru.com/viewtopic.php?f=7&t=25132
The result was strange SMART reporting (due to a "shift" in the SMART attributes), but otherwise the drive appeared to be OK.
Hopefully the following article will help you to understand the structure of HDD firmware.
The hard drive -- a computer-within-a-computer:
http://www.hddoracle.com/viewtopic.php?f=56&t=2600
There are ways to forcibly update a drive's firmware, but not by using the RARs or ZIPs found in the firmware resource dumps at places such as HDD Guru. Instead one would use Seagate's official retail or OEM updates and modify the command line that executes the update. The procedure is described in the abovementioned HDD Guru thread.
Here is my analysis of Seagate's SD1A update for the 7200.11:
Analysis of Seagate F3 Firmware Update:
http://www.hddoracle.com/viewtopic.php?f=59&t=816