Error at boot up

[mxyzptlk]

I have a problem with booting my PC when I have an external drive fitted.(has been ok until just recently).
Without the drive fitted, the boot is all ok and all works fine. I can then plug in the external drive and use it as normal.
When the drive is fitted at start up, the PC shows a disc error and starts a check of the external drive but it is so slow and eventually sticks at around 20% completed. All I can do then is power off at mains, disconnect the drive and boot up.
This only started a couple of weeks ago and I can't think of any thing which has changed. I've done the usual sweeps etc on both PC and ext drive.
Any advice appreciated

[another-view]

You shouldn't boot a PC with a USB stick or drive attached to it.

[LeMarchand]

Check the boot options in the BIOS, your external disc may be set as the first choice to boot from.