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I've noticed in using my external SDXC cards that the write speed tends to decrease over time during a large file transfer.

For example, when an SDXC card is plugged into the port on my 15" rMBP and I transfer a 90gb file from the internal SSD, it starts around 50 mb/sec and drops gradually to around 100 kb/sec.

The cards in question are SanDisk Ultra 128gb micoSDXC card, and is read using the SD card adapter. It benchmarks at around 50 mb/sec write and 80 mb/sec read. The card is formatted as exFAT with Apple Partition Map.

What causes the write speed slowdown? Is it possible to avoid?

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This does not just happen with SD or SDXC card. This is something which happens with all copies, and on almost any OS.

Basically the copy process follows these rules:

  1. Read from file into buffer (very fast)
  2. Write from buffer to destination.

What happens is that the reads fill up memory buffers faster than they get written to the SD card while the speed measures step 1 (which will need to slow down to match the write speed).

So, a slowdown is normal. 100KB/sec is rather more (or less?) than I expected though.

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