won't boot with external hdd plugged in

Yes try disabling it.

I disabled it, rebooted, no difference. Downloaded and installed WD SES as previous suggested with the same results. Could it be the hdd itself?

1) Update the specs in the "My Computer" section:

System Specs - Fill in at Ten Forums
In the left corner below in your post you find 'My System Specs'.
After clicking it you can find a link a little below that says 'Update your System Spec', click on this link to get to the page where you can fill in your system specs.
System Info - See Your System Specs - Windows 7 Help Forums

Include PSU. cooler, case, peripherals and anything attached to the computer by wired or wireless (mouse, keyboard, headset, printer, xbox, USB wireless network card, etc.)


2) Open disk management > by default some columns are compressed > widen each Status and Volume > make sure the contents within the parenthesis are in full view and that none of the characters are cutoff > view disk 0 > widen this row as needed so that all of the characters are in full view > post an image into the thread
Disk Management - How to Post a Screenshot of

3) Download and install Minitool Partition Wizard > post an image of the results into this thread
MiniTool Partition Wizard Free Edition - Free download and software reviews - CNET Download.com

4) Run HD Sentinel: (free or trial edition)
Hard Disk Sentinel - HDD health and temperature monitoring
Hard Disk Sentinel - HDD health and temperature monitoring
Post images of each of these tabs into the thread:
Overview tab
Temperature
SMART
Disk performance

Take Screenshot in Windows 10 Windows 10 General Tips Tutorials
Take Screenshot in Windows 10

Thanks Z. Give me some time to digest all of this. To me, it would be simpler to do what win7 did, allow me to decide when to restart. That way I could just eject the hdd, restart, then plug it back in.

A year or so ago I had the same problem you're having. None of these suggestions worked for me either. In my case, my external hard drive was quite old (probably at least 8 years old). I now have a couple of newer external USB drives and they work fine. (Don't know what the difference is, but it might be worth trying a new drive.)