You shouldn't need to write zeros to the disk. There is a problem that you won't get an end-of-file until you run out of disk. That's why you a program like tar...it will know when to stop reading.

I would try the raw device first (/dev/rdsk/something). It will probably work if anything will. You can also try the block device (/dev/dsk/something) but you must at least type "sync" and wait a few seconds before you disconnect the disk .