I wanting to erase a partition containing existing data by creating a new partition from gnome-disk-utility by checking "Overwrite existing data with zeroes" when formatting the partition.

To my surprise, I ran "strings" on the just erased partition and I could still see old data.

This is a on Fedora 23 with udisks2-2.1.6-1.fc23.x86_64.

To reproduce:

# write some test data
yes sensitive_data > /dev/sda4
# status before the erase
strings /dev/sda4|grep sensitive_data|wc -l
35791394
# now erase the partition with zeroes
# status after erase
strings /dev/sda4|grep sensitive_data|wc -l
544492


I ran strace on the udisksd process and I can see the writes to the drive:

27221 open("/dev/sda4", O_WRONLY|O_EXCL|O_SYNC) = 14
27221 ioctl(14, BLKGETSIZE64 <unfinished ...>
27221 <... ioctl resumed> , [536870912]) = 0

27221 write(14, "\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0"..., 1048576) = 1048576

The partition is 536870912 bytes (512MB) large. There are 512 calls to write() with a buffer of 1048576 bytes. So in theory all of the partition should be overwritten (1048576 * 512 = 536870912), so I'm surprised to see such behavior.

The drive is a SSD from a Toshiba Portegé laptop.