Apple - How can I speed up SD card writing speed (using /dev/disk*) on a MacBook Pro?

Just ran into the same problem and it seems to be that /dev/disk* is slow because it is buffered. If you use the corresponding rdisk device (ex: /dev/rdisk1s1) you should get the speed you expect. This is apparently a BSD thing.

Example:

% sudo dd if=pi.bin of=/dev/disk1 bs=1m count=4095
^C408+0 records in
407+0 records out
426770432 bytes transferred in 1393.452305 secs (306268 bytes/sec)

% sudo dd if=pi.bin of=/dev/rdisk1 bs=1m count=4095
4095+0 records in
4095+0 records out
4293918720 bytes transferred in 378.669512 secs (11339489 bytes/sec)

Ref:

• Note on Dev Entry Access

• Post on Apple mailing list

It could well be that the SD-card is that slow.

Take a look at this benchmark (Random Write, 4 KB (QD=1) [MB/s]) and you will see that most SD-cards have a lousy write performance (below 100 kB/s) for small files - and a typical linux installation consists of a lot of small files been written to disk.

The internal card-reader should be capable of reading/writing at least 20 MB/s - it's very unlikely that another card-reader will improve your experience with this very SD-card.

Once all the files are written, i.e. the system is installed, you may actually be able to run Linux from the SD-card in an acceptable speed - as it's mostly read access.