That question is a little too open-ended to be answered effectively, but I'll try anyway.
Sager is known for making DTRs (desktop replacements) and cutting edge gaming machines. For this, the emphasis is power, not battery life. So, for most Sagers with 17" screens, P4/A64 X2's, discrete GPU packages, you are looking at a battery life of 45minutes - 1.5 hours. The 9750, 9890, 9880, 5950 fall into this category.
If you replace the processor with a Core Duo, you will get about 2-3 hours of battery life, depending on work load.
Replace the 17" with a 13" screen, and you can add another .5-1 hour to the battery life.
Replace the discrete graphics package with an integrated card, and you get another .25-.5 hours on battery life.
If you want to know how to replace the hard drive, you need to look up the manual for each specific laptop. Once good resource is if you go to the Sager Reviews and Benchmarks Forum (http://www.notebookforums.com/forumdisplay.php?f=3
) and look up the reviews done by PCT, they will break down the laptop and show you the internal components. This will give you some idea as to difficult it is to open up the computer.
Honestly, almost every new model that comes out has a different chassis with different internal configurations. However, in general, it is not that hard to replace a hard drive on a laptop. For most companies, you will either have a quickrelease or will have to remove 1 screw.