The SI differs from some of the older systems in that it has definite rules governing the way the units and symbols are used. The name of a unit always starts with a small letter, unless it is the first word in a sentence, in which case normal capitalisation applies. This is true even if the symbol uses a capital letter. For example pascal and Pa are correct. The case of the letters in a symbol is important.
Computer clocks are typically MHz.
A mHz computer is a very slow one. The symbols are printed in upright Roman type always, and the singular and plural are the same. A space should be left between the numerical quantity and the unit symbol. The unit of measure is the metre , not meter . The latter is a device used for measuring things.