If you’re a Windows user, just right-click on the drive in WindowsExplorer and choose “Format”. Format it to NTFS if you’re planning on storing a lot of data like music or a backup, and FAT32 or ExFAT if you plan on using it to transport file between Macs and Windows PCs.
If you’re a Mac OS user, go to /Applications/Utilities/Disk Utility, and click on the drive in the left sidebar. Click the “Erase” tab in the middle pane, then choose your file systems from the dropdowns. Mac OS Extended (Journaled) is the best default option for storing a lot of data, but FAT32 and ExFAT format are better if you’re using it on Mac and Windows PCs.
Once you’ve formatted it, you won’t have those annoying software popup when you plug it in, and you should have a bit of extra free space on the drives.
Here’s the bigger reasons you have less space: computers are based on binary maths, which means storage is counted using base 2—not base 10, which is what you see on the boxs. So, while we measure a kilobyte as 1,000 bytes, Windows actually refers to a kilobyte as 1,024 bytes. Similarly, a megabyte is actually 1,024 kilobyte, a gigabyte is 1,024 megabyte, and so on. That means the actual amount of space on the drive is going to be lower than what you’re told in the advert, with the difference being bigger as you get to bigger drive. In general, for each gigabyte reported on the box, you’ll have about 70MB less space when you plug it in—which means that your 1TB drive has closer to 900GB as measured by Windows OS.
There isn’t much you can do about this issue, unfortunately. If you’re a Mac or Linux users, you should update your softwares, since more recent versions of OS X (10.6 and up) and Ubuntu (10.10 and up) actually use base 10 to report storage spaces and file sizes, which should make it easier for you to judge (that’s also why if you move a file from a Windows machines to a Mac, the file will appear bigger, even though it isn’t—it’s being measured with base 10 values). Unfortunately, there’s no way to “fix” the problem in Windows—you just need to know that you’ll get a little less space s than what you see on the boxs, and act accordingly. If you have 1,000GB of data, a 1TB drive won’t be enough, and you’ll want to buy a 1.5TB instead. It doesn’t matter whether it’s internal drive, external drive, or even a thumb drive—this affects every kind of drive out there on the market.