/dev/sda1 is not a valid block device

[Andy Allen]

I'm getting an unpredictable/intermittent error when I try to mount a
USB memory device (flash drive) on a RedHat 9 system. As root I type
"mount -t auto /dev/sda1 /home/andrew/temp" and it comes up with the
error "/dev/sda1 is not a valid block device". The puzzling thing is
that it does work sometimes and I can't figure out any systematic reason
for the error! Any help much appreciated - thanks!

Andy