Transfer speed of USB 2.0

You are ignoring all the overhead needed to transfer and verify that data. You don't just 'pour it down the data pipe', and hope it al arrives, without errors. Like all serial communications, it is sent in packets, with handshaking & error detection.

It doesn't answer your question, but something to watch out for. USB

Low speed 1.5 Mb/s (I think)

Full speed 12Mb/s

High speed 480Mb/s

So, a device which claims to be 'USB 2.0 full speed is no faster than a USB 1.1 device. USB 2.0 does not require a device to be fast, a keyboard operating at 1.5Mb/s can be USB 2.0 complient.

The total time to copy the files will never reach the raw transfer speed of the interface. Copying a lot of small files will take longer than copying a few large files of he same total size. Some hard disks are faster than others. If the source files are badly fragmented this will slow things down. If the files are compressed they will have to be de-compressed, if they are being copied to a compressed volume they willl hace to be compressed. The processor may be busy with other background tasks. Anti-virus software can slow things down. There are many factors which affect how long the copy will take.

No, that's the raw signalling bit rate. Actual throughput will be considerably less, due to protocol overhead and polling delays.

The best *actual* transfer rate I've seen for a USB2 external disk is about 30 MB/sec, or 240 Mb/sec.

Firewire400, despite having a lower bit rate, actually manages a slightly higher transfer rate with much less CPU overhead. But you'll need Firewire800 to come close to the drive's real transfer rate, which is probably around 80 MB/sec. eSATA should also be able to transfer at essentially drive rate, if you have a controller and external drive that support it.

Only if you could get 480 Mb/s throughput, which you can't. Using a more realistic value of 240 Mb/s, it will take 3 hours. That's for copying large files, where file lookup and creation overhead is small. Copying a drive full of many small files with Explorer will take longer yet.

Anything faster than 3 hours *is* faster than USB. However, your internal drives ought to be able to do 1.5 hours or less if they are on separate controllers, so they can be transferring simultaneously. (Again, this applies for copying large files, not zillions of small ones).

Dave

| The best *actual* transfer rate I've seen for a USB2 external disk is | about 30 MB/sec, or 240 Mb/sec. | | Firewire400, despite having a lower bit rate, actually manages a | slightly higher transfer rate with much less CPU overhead. But you'll | need Firewire800 to come close to the drive's real transfer rate, which | is probably around 80 MB/sec. eSATA should also be able to transfer at | essentially drive rate, if you have a controller and external drive that | support it.

I'm getting about 25 MB/sec (200 Mb/sec) on my WD 500 GB external drives attached via USB 2.0.

I'm getting about 40 MB/sec (320 Mb/sec) on my WD 1 TB external drive attached via Firewire-400.

I'm getting about 54 MB/sec (432 Mb/sec) on my WD 320 GB internal drives attached via EIDE. I have 2 such drives on the same computer and when I run both in parallel at the same time, on separate EIDE channels/cables, the speed per each drops to 38 MB/sec (304 Mb/sec) giving me a total EIDE speed of 76 MB/sec (608 Mb/sec). I don't have a setup to test having both on the same channel/cable together as master/slave but I would expect that to be much slower.

I'm getting about 96 MB/sec (768 Mb/sec) on my WD 750 GB SATA internal drives attached via SATA-II. I have 4 such drives and when I run all 4 in parallel at the same time, the speed per each drops to 92 MB/sec (736 Mb/sec) giving me a total SATA controller speed of 368 MB/sec (2944 Mb/sec). I could have a really fast filesystem if I use an interleaving RAID arrangement on these. But these drives are destined for other computers later on.

I'm getting about 3.3 GB/sec (26.4 Gb/sec) when copying /dev/zero to /dev/null. If I compress the data with gzip, the speed drops to 86 MB/sec (688 Mb/sec) but the CPU fan goes faster :-)