But what is it that Eraser does to clear unused space on your disk? And where this unused space can be found?
If you have not disabled the option to erase cluster tip area (generally, there is no reason why you should disable this option unless the drive uses FAT file system and is compressed; see Overwriting Properly), Eraser will start by clearing this unused space from each file on the selected drive.
When a file is loaded in memory by some application or by the operating system (or opened without file sharing), its cluster tip area cannot be overwritten and you will receive an error because of this. To reduce the amount of locked files into a minimum, you should close as many applications as possible before erasing unused disk space and even then the files locked by the operating system cannot be accessed.
After taking care of the cluster tips, it is time to overwrite the free space on the drive. If your drive is equipped with a file system that supports quota and the space available to you is limited (i.e. the space available to you is smaller than the free space on the drive), you cannot erase unused space on that drive and should ask the administrator to do it instead.
To overwrite the free space, Eraser creates a temporary directory, which it fills with files (these are deleted after the erasing is finished). Multiple files are used because it is faster than creating one huge file. Data will be written until there is no more space available on the drive. This procedure may take a long time if the free area is large and it may slow down your computer substantially; especially if the paging file (swap) is located on the selected drive. This is another reason why you should close all applications before erasing unused space.
If you are running Windows NT or 2000 and the file system on the drive is NTFS, Eraser will next overwrite the free space on the Master File Table (MFT). The reason why this is done is that on NTFS file system, clusters are not necessarily allocated for files smaller than the size of a MFT record, but the file is stored completely in the MFT (the file is then said to be resident). If you have insecurely deleted such a small file, the free space on the MFT still may contain the file body and therefore, it must be erased as well. Windows 9x does not support NTFS file system so this step will be skipped.
Finally, the names of all previously deleted (or erased) files will be overwritten. On FAT{12,16,32} partitions this is done by going through all directory entries and overwriting deleted file entries. On NTFS partitions (Windows NT and 2000 only), Eraser creates maximum length files until the unused entries in the Master File Table are overwritten.
In addition to erasing unused disk space, you can also set the paging (swap) file to be overwritten on Windows NT and 2000. Using the General Preferences window you can enable this Windows NT security feature that overwrites all unused portions of the paging file when shutting down.
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Problems? See this thread at archive.org.
