A. If when you try and compress files on an NTFS partition using Explorer (right click on a file/directory, select properties and check the compress box) the option is not available or when you try from the command prompt using the command:

C:\> compact /c ntfaq.txt /s

you get the error

"The file system does not support compression"

the cause is normally that the cluster size of the NTFS partition is greater than 4096. To check the cluster size of your NTFS partition use the CHKDSK command, e.g.

C:\> chkdsk <disk>: /i /c

The /i /c are used to speed up the chkdsk and at the end of the display it will tell you the bytes in each allocation unit:

2048 bytes in each allocation unit.
1012032 total allocation units on disk.
572750 allocation units available on disk.

If this number is greater than 4096 you will need to backup all the data on the disk and then reformat the partition using any of the following methods:

  • Start Explorer, make sure the partition is not being used, right click on the partition and select format. Set the allocation unit size to 4,096 or less
  • Start Disk Administrator (Start - Programs - Administrative Tools - Disk Administrator), right click on the partition, select format and again set the unit size to 4,096 or less
  • Format from the command prompt
    C:\> format <drive>: /fs:ntfs /a:4096

Once reformatted you can then restore your backed up data.

To understand more about the 4,096 limit please read Knowledge base article Q171892 at http://support.microsoft.com/support/kb/articles/q171/8/92.asp