When you try to boot with the drive attached then the boot process hangs.
When you plug the drive in after boot all is well.
I think you will find that the boot flag is set on the drive.... when the HP t-520 boots it scans attached media for bootable O/S.
If you're a Linux user then GParted can clear the boot flag.
Don't know if GParted is available for other O/S but there is a downloadable live GParted.
Also, and if the above does not work, you might want to turn off in the t-520 bios the boot option for other/legacy media.
Edit... hp t-520 bios settings (F10 as it boots)...... 1st boot device default is USB..... Change that to ATA Flash and Snakeoil will boot first.
Set 2nd boot device to USB (or disabled )
Save and you should be good.
