How much storage is actually in a hard drive?

You have to read the fine print on the boxes. Everything is based on the basic unit of "byte" (1 byte = 8 bits).

The manufacturers use kilobytes (kB), megabytes (MB), gigabytes (GB) & terabytes (TB) on the packaging when selling the items: 1 kB = 1,000 bytes; 1 MB = 1,000 kB (1,000,000 bytes); 1 GB = 1,000 MB (1x10^9 bytes); 1 TB = 1,000 GB (1x10^12 bytes). They're also essentially working off of the metric (base ten) system.

For the operating systems, however, they work off of a binary (base 2) system. The closest analog to 1,000 in binary is 2^10, or 1,024. The base units used for storage in the operating systems are kibibyte (KiB), mebibyte (MiB), gibibyte (GiB), & tebibyte (TiB): 1 KiB = 1,024 bytes, 1 MiB = 1,024 KiB (1,048,576 bytes), 1 GiB = 1,024 MiB (1,073,741,824 bytes), & 1 TiB = 1,024 GiB (1,099,511,627,776 bytes). So, since 1 GiB = 1.073741824 GB, a 1,000GB (1 TB) drive shows up as 931 GiB.

There are 2 sources for the confusion, however. First off, prior to 1998 the terms were essentially interchangeable: 1 kilobyte ~ 1 kibibyte = 1,024 bytes, & was usually written with the first term. From an accuracy standpoint, that wasn't a problem in the DOS & early 16-bit OS days (you know, back when you had to use special software memory managers to access more than 640 kibibytes of memory for programs, & a FPS game like DOOM would fit on 3 floppy disks), when you might be lucky to have a 100 MB hard drive. But as the scale increases, the divergence between, for example, 1 GB & 1 GiB becomes extremely noticeable (as you can attest to yourself). So, a number of international commissions in charge of regulating standards (including the use of metric/SI prefixes) essentially reset the standards: the "normal" metric/SI prefixes would apply for describing the raw, physical storage capacities of devices; & the "digital" prefixes using "-bibytes" would apply when said capacities were accessed through the operating system.

The 2nd source of confusion, however, is with RAM. JEDEC standards still use the older metric-based descriptions, even though RAM is only designed using multiples of 1,024: so when the RAM stick says it's "4GB", it actually is "4GiB", or 4,294,967,296 bytes of storage.