Multiple USB drives stability question

[Jeff Richards]

Is there a practical limit on the number of active USB drives with FreeBSD?  I've had stability issues using multiple USB drives as storage.

My initial design goal was cheap, hot-swappable storage.  I am only using a 100MB network currently so throughput on the storage is not a problem as I can't push the data to/from the drives faster than what my network requests are.  

I first tried my setup on 7.0, then migrated to a newer PC, then upgraded to 7.2. 
 
I have the following USB drive setup:

1 320GB gmirror (320x2) + gjournal + ufs2
1 1TB gmirror (1TBx2) + gjournal + ufs2
1 150GB gjournal  + ufs2

I also have another 1TB gmirror (1TBx2) + gjournal but removed it.  The system crashed when I used these drives (bacula or bonnie++) so I pulled them to test on another system.

Recently my stability issue has been when I have been writing data to the 150GB gjournal drive from the 320GB gmirror device (USB device -> USB device).  It will be working fine, then all I/O stops on the 150GB drive.  The system remains responding to other USB devices etc. for a while.  I try rebooting and the system crashes with gjournal errors (didn't write down, but I will later).  

Every time this happens the 1TB gmirror comes up fine but one of the 320GB providers is missing.  No problem after 'gmirror forget' and 'gmirror insert'.  Everything rebuilds fine.  The 150GB gjournal drive is fine after a 'fsck -y'.

I do pair the gmirror drives to the same USB adapter.  Found out after initial testing with multiple USB adapters that they do not appear standard enough to cross adapters like I would for a production server at work to prevent SPOF with an adapter.

I have tried Linux as well with softraid and LVM2 on the same hardware.  It worked fine until I applied software updates and the udev took 30+ minutes to boot.  I went back to FreeBSD.  Even when I crashed I was back up in 2-5 minutes.

I can and will provide more detail if requested.  My concern is that the issue seems to continue no matter what hardware/OS changes I try.

Thanks in advance.