I could also title this section How to Risk Your Data as I am not a fan of disk compression. If a compressed drive crashes, there is little to no way to recover the data in the compressed drive. Disk compression also adds to the overall overhead of operating your computer. The file has to be found in the compressed drive. Then it has to be de-compressed before it is finally passed back to the computer itself. Yet, I realize that not everyone has a multi-gigabyte hard drive and that space is becoming rather tight on what you do have.
Whether you are using Stacker or the Microsoft disk compression program, they all have some common elements. First, to create a compressed drive you have to understand what host means. For example, you have a C: drive that is 500 megabytes in size that is nearly filled to capacity. When you compress that drive you end up with two drives, the compressed volume and the host drive. The host drive is the actual physical hard drive. It hosts the compressed volume.
The compressed volume is nothing more than a super large file on your hard drive. Due to the way the disk compression program is written, you access that file as if it were a hard drive. It has folders and sub-folders and file names just like a real hard drive. Only difference being, it is a file written to a hard drive instead of the hard drive itself.
The first time you run your disk compression program it is going to have to create that compressed volume and then read every file on your hard drive, compress it and then write it into that compressed volume. This is an operation best done when you can leave the computer alone for several hours. You do not want to be interuptting the program by doing other things with the computer that would require any kind of disk access. Best to just start the program and walk away until it is done.
Most compress programs will tell you that they can double the amount of space on your drive. In theory that is correct. In reality, you might compress your data by 50% rather than the 100% claimed. This is due to the way various files compress. Straight text files, for example, will easily compress to the 2:1 ratio. Graphic files, like a gif or jpg picture, might squeek out a 1.1 to 1 compression ratio. Program files might get compressed as much as 1.8 to 1. Overall your actual compression ratio is going to be in the neighborhood of 1.5 to 1.
To translate those strange ratio numbers into something more understandable, let's go back to our 500 megabyte drive. You've installed so much stuff on it that you have 480 megs of data on the drive. Leaving you 20 megabytes free (which, btw, is the minimum I recommend any drive to ever have free). After you run the disk compression program, assuming you get a 1.5 to 1 compression ratio, your drive now has about 180 megabytes free. Not much considering todays bloated programs and their deep hunger for hard drive space, still, an improvement over the scanty 20 megs you did have free.
Considering the cost of hard drives today, and the fact that the manufacturers of those drives include software that allows any computer to access the full size of the drive, disk compression really isn't worth the risk to your data. A one gigabyte hard drive sells for under $200 nowadays, and the prices continue to fall.
Still, if you must use disk compression, remember to first backup your data before installing the disk compression. Then backup often after that in the event of a catastrophe. Keep in mind, as well, that compressing your drive will increase the amount of time it takes to access the data and for programs like Defrag and Scandisk to complete their tasks.