Intermittent pauses copying from one usb drive to another

[rendaw]

  Referencing 
http://lists.freebsd.org/pipermail/freebsd-usb/2011-February/009950.html
I'm not sure the best way to go about referencing old threads, but I 
hope this is acceptable.

I'm having the same issue with two 3.5" drives in USB 2.0 enclosures 
operating in USB 1 mode.  Like the OP, I see write activity for 1-5 
seconds followed by 1-2 minutes of no activity (I get the same vmstat 
output).  Unlike the OP, I get no error messages in /var/log/messages.  
I get the same behavior copying files from a local server both scp'ing 
and tar-over-ssh'ing.  Doing tcpdump shows the network traffic 
completely stop for those pause periods as well (other than some dns or 
arp or something chatter every once and a while).

I did try it on a linux box (just because it was handy) and I got full 1 
mb/s speeds with dd (dd if=/dev/whatever of=/dev/null).  I get around 
800kb/s on FreeBSD doing the same test on either drive.  I have the two 
drives in a ZFS raidz setup, but "top" says I have around 400 of 512mb 
ram free and the about 2.0 ghz processor is more or less completely 
unloaded.

I tried the same scp to the linux box's internal drive and got no 
pausing, so I don't think it's an scp/server issue.

Assuming this isn't caused by drive message incompatibility, is there 
some other configuration that could be wrong?  Are there some other 
diagnostics I could try?

Thank you very much,
Andrew Baxter