Adding USB stick prevents PiDrive booting - advice please?

Hello, I’d like some advice please on a bit of a problem I’ve encountered if I have a USB stick plugged into a PiDrive and reboot the PiDrive.
Basic setup is a 250GB PiDrive, RPi 2 in the 6in enclosure using WD cabling and PSU
HDD has been setup/partitioned using the supplied SD card and everything works well.
For reasons that aren’t important, I (hot)plugged a small 8GB FAT32 formatted USB stick into one of the USB ports on the RPi 2, it was recognised, read/writable and everything was fine (at this point it was identified as /dev/sdb) so I copied some files to it and left it in the RPi 2.
A while later, I shutdown the system, left it a while, then powered it up again (still with the extra USB stick inserted) and walked away to my main computer.
On trying to SSH into the PiDrive setup, I couldn’t get to it, no response to ping etc. so I returned and plugged a screen in and saw panic messages.
Looking at the messages, it was clear that the OS couldn’t start because it was looking for something on /dev/sda (which I assume is what the PiDrive would have been when I first set it up) and I guess that the extra USB stick had now been assigned to /dev/sda and therefore missing the required files (again I’m guessing that the PiDrive would have been assigned /dev/sdb at this point ?)
Power off, remove USB stick, and all is well again, so I think I know what the problem is (which device gets to be /dev/sda) but what I would like to know is does this mean I can’t use an additional USB storage device ?
I don’t understand how the device names are allocated - I understand about using UUID in fstab and so forth, but not how the /dev/ is handled, so I wouldn’t know where to start trying to “control” that allocation.
I guess this is more of a Raspberry Pi question than a PiDrive question, but would appreciate any pointers if anyone would be kind enough to share them?
I should also add that I didn’t have the extra USB stick inserted when I did the original setup if that would have any bearing on the subject.
Thanks to anyone who made it to the end of this ramble :blush:

I made it to the end- no problem!

I’m trying to get more info on this and will get back later. Thanks.