Decoding USB Write Speed in dd

Decoding USB Write Speed in dd | Generated by AI

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Question: What does this dd output mean? How is the disk write speed? Please convert it to human-readable format.

Answer:

This output comes from the macOS dd command (used with sudo dd if=... of=/dev/rdisk10 bs=1m), which is writing a Windows 11 ISO file to a USB flash drive (likely /dev/rdisk10 is your USB stick in raw mode for faster access).

dd periodically prints status when you press Ctrl + T during the copy (that’s why you see multiple lines). Each line shows progress at that moment.

Breakdown of one line (example):

866+0 records in
866+0 records out
908066816 bytes transferred in 43.725150 secs (20767609 bytes/sec)

records in / records out: How many blocks were read from the ISO and written to the USB.
Here each record = bs=1m = 1 MiB = 1,048,576 bytes
So 866 records ≈ 866 MiB written so far.
bytes transferred: Total data written up to now (in bytes).
in Xs secs: How much real time has passed since dd started.
(N bytes/sec): Current average write speed up to this point.

Your speeds in human-readable format (MB/s):

First status: ≈ 20.4 MB/s (20,439,149 bytes/sec ≈ 20.4 MiB/s)
Second status: ≈ 20.6 MB/s (20,572,527 bytes/sec ≈ 20.6 MiB/s)
Third status: ≈ 20.8 MB/s (20,767,609 bytes/sec ≈ 20.8 MiB/s)
Final status: ≈ 15.5 MB/s (15,542,913 bytes/sec ≈ 15.5 MiB/s)

The overall average speed dropped to about 15.5–16 MB/s toward the end.

Is this speed good or bad?

Typical cheap / older USB 3.0 flash drives: Sustained write 10–30 MB/s when writing large continuous data (like an ISO).
Good USB 3.0/3.1 flash drives: 40–100+ MB/s sustained write.
Very fast modern ones (USB 3.2 Gen 2, SLC/MLC, or premium models): 150–300+ MB/s possible, but rare for normal thumb drives.

Your speed (15–21 MB/s) is quite typical for:

An average or budget USB 3.0 flash drive
A USB 2.0 port (even if the drive is 3.0)
A drive that’s getting full / thermal throttling / fake-high-capacity fake drive

It’s not very fast, but also not unusually slow for many common USB sticks when writing ~5–6 GB continuously.
If you want faster ISO → USB creation (2–4× faster), try:

A better USB flash drive (look for ones advertising 100+ MB/s write)
Confirm you’re using a real USB 3.0/3.1 port (blue port usually)
Use tools like Balena Etcher or Rufus (they often achieve better performance)

References:

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