Earlier could copy data on mac-dos(fat) drive now suddenly can't.

the Drive you are using is a Windows does format & you cannot use it to copy things from your mac to it because the directories dont match & it gives a too large volume error. you should probably backup the data & re-Format the Hard drive to Mac OS extended (Journaled).

THanks,

Convert the drive to NTFS and use (buy $19) Paragon NTFS for OS X.

it is not copying even a 8 gb file

I believe (from my possibly faulty memory) the file size limit for that format is 4GB file size.

Folders are an illusion created for you by the Finder or similarly by Windows. A folder consists of one more files for the folder itself, plus a mess of flat files that are shown (to you) as "inside" the folder. Each file is still subject to the individual file size limit, you do not "add them up" to get a new file size.

¿ Are there any individual files over 4GB in that folder you successfully copied?

Convert the drive to NTFS (or even HFS+) non-destructively with one of Paragon-Software's drivers. If you use Windows then use the convert fs would be one way.

Or exFAT.

http://technet.microsoft.com/en-us/library/bb456984.aspx

Click Start, point to All Programs, point to Accessories, and then click Command Prompt.

• At the command prompt, type the following, where drive letter is the drive that you want to convert:

  convert drive letter: /fs:ntfs

  For example, type the following command to convert drive E to NTFS:

  convert e: /fs:ntfs

  Note If the operating system is on the drive that you are converting, you will be prompted to schedule the task when you restart the computer because the conversion cannot be completed while the operating system is running. When you are prompted, click YES.
• When you receive the following message at the command prompt, type the volume label of the drive that you are converting, and then press ENTER:

  The type of the file system is FAT.
  Enter the current volume label for drive drive letter

• When the conversion to NTFS is complete, you receive the following message at the command prompt:

  Conversion complete

• Quit the command prompt.

And free NTFS <=> HFS+

http://www.paragon-drivers.com/ntfs-hfs-converter/

if I format it with Mac Os extended can I use it on windows later ??

Windows does not believe that there are any other Operating Systems in existence. Using any non-Windows File format requires a Windows add-on. The most popular for Mac OS Extended under Windows is MacDrive.

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Windows New Technology File System (NTFS) is so complicated, poorly documented, and out of Apple's control, that Apple opted to support only Reading, not Writing that format. As the hatter points out, an inexpensive Software Driver from Paragon seems to work very well, allowing the Mac to Read and Write that format reliably.

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Apple software Geeks are not just being jerks about "not Invented Here" when they decline to support NTFS. In sharp contrast, they are adopting the Well-documented Windows Server Message Block v2 (SMB2) as their Internal Server-to-Mac format going forward, replacing their own Apple File System (AFS) for Server to Mac transfers.

The latest Paragon HFS is fully as easy as any mounting, reading, and writing to HFS+.

HFS+ and disk drives, I actually trust and use Windows for better support of storage, and when needed to recertify drives.