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Picked up one of these and it seems to do the job.

Good price, but no eSATA.
Generally, eSATA is twice as fast USB 2.0
Once you try eSATA for your external drive it's hard to just use USB 2.0

#2, are they really?
I have 2 usb externals, one is a WD sata drive that I put in an antec case, the other I just got and it is a western digital my book 1.5 gb triple interface that I am using usb until I buy an esata cord. both write at 7-15MB/s.

Since usb is around 480mb/second it seems like the rotating disk is the limiting factor for big reads/writes. since my speed is so much slower than the usb speed, is usb really the bottleneck?

I hope esata will be much faster. what kind of speed should I expect?

#3:

1) Your drives, the enclosure, or your system's USB subsystem are a bottleneck that has nothing to do with either "rotating disk" bottlenecks in general nor USB2 bottlenecks in general, nor USB2's max theoretical bandwidth.

2) Plenty of external enclosures over USB2 with modern hard drives manage sequential large file transfers, and benchmarks, at a bit over 25MB/s write, 35MB/s read.

3) USB cannot transfer 480mb/s, period. That is the bus hypothetical limit if there were no other overheads involved, a little under 40MB/s is the normal cap on USB2 transfers.

4) Current generation SATA300 hard drives can easily attain over 70MB/s write and 90MB/s read over eSATA without the system overhead including interrupts and CPU utilization of USB2. That's large sequential access, but even if we want to talk about small or more random access the relative difference remains.

The speed you should expect with eSATA is the full speed you'd have with an internal SATA connection, it's not just 35-50% faster, it's closer to 100% faster.

However, if you were going to buy a PCI eSATA card (rather than PCIe or just a bracket that extends your internal SATA to an external eSATA socket), a PCI eSATA card will be bottlenecked by the PCI bus. It will still be significantly faster than USB2 unless you have other high bandwidth PCI devices concurrently used, but would then be closer to 35-50% faster rather than 100% faster at large sequential files, but probably still twice as fast at small random access.

#5, thanks for the info.
I have a skinny acer aspire that was a $239 black friday deal last year.
It comes with an esata port. I am assuming that is off the motherboard and not off a pci card.

With this would any of the following externals eSatas be decently speedy?
$75 1 GB fantom
blacX or vantec esata dock
WD 1.5TB mybook using the esata

or is the speed limitation likely to be caused by the Acer computer (motherboard/cpu)?

thanks for any help