You want to check the file partition on the external HDD.  If it is  exFAT, you will need to format it to NTFS.  It might help to resolve the issue.
My dell XPS 8500 gets stuck on dell logo every time I start up the computer.   If I remove external HDs I'm able to boot the computed normally, but it's a temporary solution. If I reboot with the external HDs connected it get stuck on the dell logo again.   How can I permanently fix this issue?   My bios is the A14 set to legacy. I already replaced the cmos battery and run the diagnostics with no problems.  
XPS 8500 Stuck on Dell Logo