Dd to wipe partiton, data gets read after write?

Hi,
want to wipe a partition on my hdd. Doing this with dd if=/dev/urandom of=/dev/sdxx and get ~10MB/s (it's a WD green 2TB in USB2 external case). I tried different bs values (2048, 4096, 1M, 4M, default) but nothing helped.
Now I saw in conky that mot of the time read and write speeds are identical. So I assume that the data gets read after write, is that possible? I also tried it with shred, same behavior on read/write except it's a bit slower ~8MB/s. Perhaps this has to do something with USB. Of course I could connect with SATA directly for wiping but the problem then stays for later when I want to use it as external device...
Any suggestions?
Thanks in advance.

Thanks for these replies.
@ROOKIE Mh, thats not the case since the data gets read fom /dev/urandom.
@brebs Doesn't know that there are settings/drives supporting that. Good to know but it says "write-read-verify = not supported"
@teateawhy Lready done with urandom but always good to know that this exists.
So I'm still curious whats going on. And now I can say that it still happens with filesystem on that drive while copying files.
Got read = write = 30MB/s copying files from internal SATA connected to external USB 2.0 connected HDD. So overall 60MB/s what I assume it should be but just writing...
Same when using dd with /dev/zero as source just slightly faster.
Finally reading speed is ~85MB/s which seems to bee the USB 2.0 limit, so this works like expected.
Thanks again and perhaps someone can (help me) solve this.

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