I may be wrong but from the code that you present it appears that you have failed to direct the output 'of=' to /dev/sdb1 your output command should be of=/dev/sdb1/backups/backup.iso.
I don't understand why you are using the extension '.iso' since this is a file system for CDs and DVDs and I would suspect that both your sda1 and sdb1 are formatted as ext3 or ext4. The 'normal' use of the 'dd' command is to name the output file as a '.image' file, not that it makes any difference, in linux, what the file ending is - it is the first characters of a file that determine what or how the file can be read.
So unfortunately it appears that you have written, or at least tried to write 250GB to a file on your sda drive. I somehow doubt that you were successful since even if it worked it would have taken about 2 and 1/2 hours minimum.
If you seriously want to duplicate (clone) your sda drive to a file on your sdb drive then I would suggest that you boot your machine with a liveCD so that your sda drive is not mounted.
By the way what distro are your using, and does it still boot.
You may want to read http://www.linuxquestions.org/linux/...ything_With_DD
before proceeding further.