What seems to happen if you plug in a USB disk and boot up the computer is that the USB stick gets assigned /dev/sda and the internal harddisk gets assigned /dev/sdb .
This is different from a boot without the USB stick inserted, your internal hard disk will get assigned /dev/sda in that case. And this is why you are getting the boot error - LILO is telling the kernel to look for /dev/sdb1 as the root filesystem's partition but there is no longer a /dev/sdb ...
What you can try is boot the computer and at the LILO prompt type the following before pressing ENTER (I assume that "linux" is the name of the label that you used to boot the default kernel configuration, adapt it to your real situation):
This "root" assignment should hopefully be carried forward by LILO to your kernel and your computer should boot normally. If it boots, you login, and then you should immediately edit the file /etc/lilo.conf to replace all occurrences of "/dev/sdb" with "/dev/sda" and when you have done that, you need to run the command "lilo". That will fix the issue permanently.
If adding the "root" parameter does not help, then there are some more things to try but I'll keep that for later.