Does transferring files on a home network = higher charges by an ISP ?

I have a home network of 3 computers which are all linked through a modem supplied by my ISP.

I know that every time one of those computers uses the internet to download or upload files the ISP charges for it.

I however use the home network to back up files already downloaded by transferring them from one pc to another via the common network.
Some of these files are quite huge in size.

My question is this. Am I incurring extra download/upload charges for using the modem to transfer files between the computers?

I have email my service provider to ask them , but they could not answer the question, instead they sent me a link to "most common asked questions abouts download charges" which doe not address this issue.

Can anyone please advise me if it may be cheaper to transfer the files through an external drive such as a USB stick?

 

I cannot speak for your ISP but since you aren't downloading anything from the internet via their network it is very unlikely you'd incur charges (I won't say 100% but I'd go as far as 99.99% sure ).

EDIT:
Personally I hate pay as you use ISP's (though I do use one via mobile/cellular when out and about). Can't you get a fixed pay per month provider down under?

 

Yes I am with the local national phone company here and a sI have the internet, mobile phones, land line and pay tv all bundled under one plan with them, I am only paying $Aus $ 40 a month for 50gig a month. If I exceed 50 gig I get slowed down to dial up speed but do not get charged any more for teh month Quite a reasonable plan actually.