A 500GB drive shows less because 1000 and 1024 are both in use. The two kinds of gigabyte, bits vs bytes, real usable space and how to read unit labels.
Storage manufacturers count a gigabyte as a billion bytes, while operating systems have long multiplied by 1024. A device labelled 500GB therefore shows as around 465GB when connected. Nothing went missing; the counting differs. Precisely, 500 billion bytes divided by 1024 cubed (1,073,741,824) is about 465.66, and a 1 TB drive shows as about 931 GiB.
Computers work in binary, so powers of two are natural units. Two to the tenth is 1024, close enough to 1000 that early usage borrowed the kilo prefix. A standard later introduced separate names for the 1024 basis to remove the confusion, but in practice both remain mixed. The 1024-based names (kibi, mebi, gibi, tebi) were set by the International Electrotechnical Commission in 1998. Even within one operating system, though, memory is often given in 1024 units and storage in 1000 units.
Internet speed is usually written in megabits per second while file size is in megabytes. A byte is eight bits, so they differ by eight. That is why a 100Mbps line downloading at around 12MB per second is normal. Feeling that speed falls short of what was advertised often comes from confusing these units. Downloading a 1 GB file over a 100 Mbps line takes 8,000 megabits divided by 100, or 80 seconds in theory; in practice protocol overhead and congestion make it take a little longer.
Beyond the notation gap, usable capacity is smaller still. The file system's own structures, a recovery partition and pre-installed software all occupy space. That is why a new device is not empty from the start. Because files are stored in blocks, the smallest unit of storage, even a one-byte file can occupy several kilobytes on disk, so the more small files you have, the more space they actually use beyond their listed total.
Knowing rough magnitudes makes planning easier than memorising numbers. A photograph is a few megabytes, a song a few megabytes, and high-quality video tens to hundreds of megabytes per minute. That much feel is enough for most decisions about backups or transfer times. Photo and video sizes vary several-fold with resolution, compression and quality settings, though, so when you need a precise figure, saving a few real samples and averaging them is most accurate.
A single upper or lower case letter changes the meaning, so read labels carefully. A capital B is a byte and a small b is a bit. Kilo is officially a lower case k, but a capital K is common, and any label with an i explicitly marks the 1024 basis.
When choosing a drive or a cloud storage plan, leave headroom rather than looking only at what you use now. Around 7 percent disappears to the labelling difference, the system and preinstalled apps take space, and drives often slow down as they fill up. As a general rule, about 1.5 to 2 times your current need gives comfortable room for a few years. A backup drive must be larger than the original, and keeping backups from several points in time needs more again.
Many complaints about data capacity come from labelling differences rather than real faults. The situations below are mostly normal behaviour.