Essentially (it is actually a bit more complicated), the information on a hard drive (or solid state drive - a computer will normally treat them basically the same, and actually floppy disks, flash drives and other removable media are similar) includes:
This is what takes a continuous substrate and splits into sectors for use by an operating system. In ye olden days, the operating system or special utilities would perform this task. In modern drives, the drive itself takes care of the bulk of this. And in SSDs, the drive is always a giant array of bits, so there is no true formatting to be done, though plenty of similar activities go on behind the scenes (entirely within the drive) for wear leveling and other purposes.
The partition table is a standardized area near the start of a drive that defines the areas of the drive - both size and function. This is what makes the difference between a drive being one drive (typically C: for DOS/Windows, but that is totally arbitrary), two drives (e.g., C: and D:), a Linux partition, FAT16 vs. FAT32 vs. NTFS, etc. If you change a drive from C: to C: + D:, the key change is in the partition table. If you have a drive that was copied from a smaller drive to a larger drive, you are only able to make use of the additional space if you modify the partition table.
This is a very small piece of software stored at the beginning of a partition that tells the computer how to start up. It is irrelevant for a secondary drive, but absolutely crucial if the drive is used to start your operating system (DOS, Windows, Linux, etc.) It can be corrupted by viruses, messed up due to overuse (not an issue with hard drives, but a real concern with floppy disks), etc.
The bulk of any drive is used for data. The data area is divided into directories containing file details (name, creation/modification time/date, size, permissions) and the actual files. The details can get incredibly complex and have evolved from a single directory and no security (CP/M, first versions of MS-DOS) to hierarchical directories, multiple levels of permissions and many other features.
So where does that leave you? Windows has a very well-defined structure for the primary drive. Normally this is C:. It includes directories with specific names - \Windows, \Users, \Program Files (x86), etc. All of that matters for use as the primary drive. For a secondary drive, the low-level format and partition table need to be set properly, but not much else. My general recommendation is:
- If the drive is more than a few years old, stick it on a shelf (unless you want to take the time to do a good destructive rewrite) and don't use it in a new machine. Hard drives (not SSDs) are mechanical and will fail - it is just a matter of time. At the current price of SSDs and removable flash drives, it is simply not worth the risk of relying on an old hard drive, plus SSDs (external flash drives not as much) are much faster than most hard drives, especially older hard drives.
- If the drive is relatively new, reformat it and use it as one large partition as a secondary drive. This wipes out Windows (which is on your primary drive, by definition) and all programs (which you have already installed on your primary drive) and gets rid of any other corruption (e.g., viruses, malware, etc. that had been undetected previously). At the same time, a good format (one that checks every sector for readability) will spot certain types of problems that might otherwise have gone undetected until they spread to the rest of your drive. If you do get any errors when reformatting, toss the drive. It is simply not worth the risk to your data. No software installation needed on the new drive - it can be used as a secondary drive on (almost) any computer simply by plugging it in.
Consider that at current prices (e.g., my local Micro Center has 64 GB USB 3 flash drives for $7.99) you can backup tons of data for literally pennies. It just isn't worth it to reuse a hard drive more than a few years old. That's $7.99 for 64,000 Megabytes. When I started in this business, $10.00 for ONE MEGABYTE (e.g., $200 for a 20 Meg. hard drive) was a decent price.