Resuming a DD of an entire disk

As @don_crissti already commented, just use seek= to resume.

dd if=/dev/urandom of=/dev/disk/by-uuid/etc bs=512 seek=464938971

GNU dd also supports seeking in bytes, so you can resume exactly, regardless of blocksize:

dd if=/dev/urandom of=/dev/disk/by-uuid/etc bs=1M \
   seek=238048755782 oflag=seek_bytes

A larger blocksize should help with speeds even for a slow device like /dev/urandom.

If you are looking for faster alternatives, you could cryptsetup plainOpen with a random key and zero that, it should beat /dev/urandom by an order of magnitude (without AES-NI) or even run at full speed (with AES-NI).

You could also use shred -n 1 if pseudorandom data is good enough for your use case. shred should be able to utilize the full disk speed, even on a very slow machine.

Just a reminder for people who would like to copy rather than just randomizing disks (which is not that common) : you can use skip=BLOCKS to start reading at the proper position, and seek=BLOCKS to start writing at the correct position. Both options use blocks, not bytes. When breaking/restarting, it's advisable to remove a bunch of blocks just in case. It is usually worth raising the bs value above 512, as you can reach better performance if you read a lot of data in a row.

In your case, it is indeed a block value that you need to pass to seek. Maybe you should try to adjust bs to see if you can enhance speed, as /dev/random should go fast (pseudo-random and non-blocking when it has no entropy available)