USB booting :: Unable to install 2.1 to USB Drive

Sounds to me like a timing issue.
Do you have a floppy drive on this system?
If so try the bootfloppy-usb, it has a slower boot routine just for such case.
Also, the usb ports on the back of the PC generally are faster to register than front ports.

1) no floppy on system
2) USB (2.0) 4 port hub connected to USB (2.0) at back

OK. Please try this:
1. Download bootfloppy-usb.img
2. As root mount bootfloppy-usb.img /mnt/test/ -o loop
3. copy over the /mnt/test/minirt24.gz to the pendrive

Let me know.

Sorry to bug you again but I have got /mnt/test/minirt24.gz but am unsure where on the pen drive I should copy it to... (btw I have 2 partitions on the drive & I have put it in the /sda2/myapps folder but this has had the effect of now taking 4 attempts to boot into dsl 2.1)

thanks again for your help

You are replacing the current minirt24.gz on the pendrive.
If booted from the pendrive, it is /cdrom/minirt24.gz so as root
cp /mnt/test/minirt24.gz /cdrom/.
if not then mount the pendrive, typically /mnt/sda1 and
cp /mnt/test/minirt24.gz /mnt/sda1/.

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