Well.I myself didn't know.But ,after looking your question,I browsed through the internet and found the answer to your question.

double = adjective

twice = adverb.

So what is the practical utility of knowing that double is adjective and twice is an adverb.

Here is the practical utility is :you can say twice as many as or twice as much as nut you cannot say double as many as.

But why?

Here is the explanation

As per oxford dictionary much,many are all adjectives.

as when used for comparison acts as an adverb.So it is correctly modifying the adjective much/many.

twice is an an adverb.By definition an adverb can modify another adverb,verb,or an adjective.But an adjective can only modify a noun.

So twice [adverb] correctly modifies as [adverb],which in turn correctly modifies the adjective many/much.

For eg: My marks are twice as many your's.

You can also say : There were twice as many books in the library as in my home.

See books =noun
in the library = prepositional phrase

the adjective many correctly modifies the noun books.

This is the reason why you say ;He is as tall as me. [tall adjective] [as adverb] and not He is as much tall as me.

Back to your example.

You cannot say double the hard drive space that it did in 2004 because the hard drive modifies space (a noun).Hard drive specifies what kind of space it was.Now you can modify an adjective (hard drive) with only an adverb twice and not an adjective double.

Hence twice the hard drive space that it did in 2004.

Inherently,there is NO difference in meaning between double and twice both means two times.

An example where double is used correctly for comparison.

20k people voted in this year's elections,double the number in 2006.

Number-noun
double-adjective

You can't use twice here.

If you have to use twice,then the correct sentence formation would be.

20k people voted in this year's elections,twice as many people as in 2006's elections.

people-noun
many-adjective
as-adverb
twice adverb

The reply above is mostly correct.

As for the GMAT, you will see 'double, triple, etc. mostly in verb form, Twice as many/as much will depend on whether what follows is count or non-count.

More frequently though, the issue of twice, three, four times more, etc,- this way of expressing quantity is considered redundant. What is preferred is double/twice, triple/thrice, quadruple/(quadruply). We see this especially with expressions like: "...multiplied two times...". This should simply be changed to the more concise 'doubled' (because it is a verb). In non verb situations, you would probably use 'twice as many/much as...'

is it countable than twice and non countable double?

u should post the whole question

Just remember when countable then double, here it is $200
when uncountable then twice.

ok i have written wrongly.

This is an assumption that double is used when it is uncountable and twice when uncountable.

There is no such thing as this.The oxford dictionary,world's most credible grammar source,makes no such claim as this.

The form many is used when it is countable
much is used when the noun being modified is uncountable

According to your logic,the following sentence would be incorrect,because you can't count rush can you? 1 rush 2 rush can can you?

But the sentence below is grammatically correct.

This morning,there was twice as much rush on the subway as is on other days.

See I used much-used for uncountable nouns.

The reason

What do you say? Much money or many money?

much money is correct.


Now using twice correctly with a countable noun.

There are twice as many cars on roads today as were two decades ago.

Can you count cars? Yes
1 car 2 cars 3 cars.

Another example

In this year's election,Candidate X received twice as many votes as did Candidate Y.

Can you count votes? A big YES.If you couldn't you could never determine who won the elections.
1 vote 2 votes etc.
Here is another correct sentence

Wife :Why are you late today?
Husband: Honey,cool down.There was twice as traffic on road today as is on other days./There was twice as much work at office as is on other days.

See work and traffic are uncountable nouns.I have busted your myth
Try using many in the example.

This morning,there was twice as many rush on the subway as is on other days.

Native english speakers can tell by using ear that the above sentence is weird.The reason -many is countable.


As to whether double is used for count-ables.The following example will bust that theory too.

This year the company earned $1M in profit,double the profit earned in last year.

Here double is modifying -profit[noun]
earned last year is modifying the profit[noun].Therefore,it acts as a noun modfier or adjective.

The entire phrase-double the amount earned in last year is an appositive or as MGMAT calls it noun phrase or absolute phrase.

You may be thinking double is modifying $1M.This sentence will clear those doubts.

This year the company earned double the profit it earned last year.

Can you count profit as such ? 1 profit 2 profit NO!.
Dollars is unit of measurement of profit.