[Info-vax] USB disk enclosures

[Hans Vlems]

My DS10 at home runs axp/vms v8.4.
It has a chinese USB interface that is recognized by VMS. Memory sticks of various sizes work well and are recognized with their specific DNA device number. 
Enter two 2.5" SATA disks, 80 GB and 120 GB respectively. I bought two identical disk enclusures put the disks in and did an init/system on the 80G drive on the DS10. Mounting worked well and I copied 40GB of disk images to it.
Dismounted the DNA device. Disconnected the disk and replaced it with the 120G disk enclosure. 
It got the same DNA device number. Init/system resulted in an error message that the number of blocks was different from the previous time the device was mounted. A backup to it failed and the DNA device was in mount verification state.

Apparently the same enclosure hardware for two different disks, both make and capacity confuses the USB drivers.

I connected both enclosures and sho device returned one DnA2: device while UCM two disks were shown, both called DNA2.

Is there a workaround for this, so that I can use them both on the same system, at least alternately?
Hans