Is a megabit the same as a megabyte?

No, and the gap between them is a factor of eight. Connections and link rates are quoted in bits per second, files and storage in bytes, and mixing the two is the commonest arithmetic error in this whole subject.

One letter separates the two units and a factor of eight separates the quantities, which is why so many disagreements about speed are really disagreements about notation.

Where each unit belongs

Bits per second describes anything that moves data along a link. A broadband line, an Ethernet port, a Wi-Fi radio, a fibre module, a SATA bus and a PCI Express lane are all quoted this way. The unit is historical and it is universal: it is what the standards define.

Bytes describe stored things. A file, a drive, a memory card, the free space on a volume. Eight bits make a byte, and a byte is the smallest unit most software counts in.

So a rate in bits and a size in bytes are describing different things in different units, and comparing them without converting produces an eight-fold error in whichever direction you happened to guess.

The notation, and why it does not save you

The written convention is that a lower-case b means bits and an upper-case B means bytes: Mb against MB, Gb against GB. Applied carefully, that is enough to tell them apart at a glance.

It is not applied carefully. Packaging, retail listings and interface labels use the two interchangeably, capitalise for emphasis rather than for meaning, and sometimes drop the b altogether. The test that always works is to ask what is being described. A link is in bits. A file is in bytes.

The other prefix problem, which is separate

Once the bit-or-byte question is settled, a second convention appears and it is a different problem entirely. Storage makers count a terabyte as ten to the power of twelve bytes, in the decimal sense the prefix has everywhere else. Many operating systems display in units of 1024 instead, and often keep the decimal-looking suffix while doing it.

That is a matter of which power is being used and it changes nothing about the drive. Bits against bytes is a factor of eight; decimal against binary prefixes is a different factor again, and the two are frequently blamed for each other.

Where the confusion actually costs something

Judging a line against a copy. A download running at a figure that looks like an eighth of the advertised line rate is running at the line rate in the other unit.

Sizing a backup window. Working out how long a large backup takes over a link means converting once, at the start, and then thinking only in the storage unit.

Comparing an enclosure to a drive. An external enclosure’s host link is quoted in bits and the drive inside it in bytes per second, and the two have to be brought into the same unit before either can be called the limit.

The habit worth having

Convert once, immediately, and write the unit down beside every figure you have collected. Two numbers on a page without their units are not comparable, and in this subject they are usually eight times apart.

The manufacturer’s own documentation comes first, and this page is not a substitute for it. Every rate, capacity, class figure, endurance rating, port speed and power figure on this page is reported from the manufacturer's own documentation, with its unit written as published, and named as such — none of it is measured here. A stated capacity is reported as the maker counts it, and is never restated in another unit, because the two are not the same figure. And the statement repeated rather than assumed: a class figure such as AX3000 or BE9300 is the arithmetic sum of the theoretical peak rates of separate radios, a quantity the standard defines rather than one a link produces. No single connection reaches it, and adding the bands together is exactly what makes it large. What a given link carries is settled by the slowest part of the chain, which no comparison page can know.