All the BIOSes I have seen, never display ‘Removable’ for a removable drive.
Booting as ‘USB HDD’ is normal, even for a USB Removable flash drive.
If possible, never configure your BIOS to boot from the USB drive as the first hard disk. This can cause issues with some OS’s that you are installing and they may install boot code onto the USB drive instead of the internal hard disk. Always use the BIOS Selection pop-up menu to select your USB boot drive (usually you press ESC or F8 or F12).
I suspect in your case, the USB drive was not correctly prepared by Tuxboot (often these programs don’t install bootcode into the MBR and assume it already has MBR boot code, or they don’t set the partition as active - they just format the partition and install PBR boot code into the partition). So your BIOS was trying to boot from the USB drive, found that it had no valid boot code or bootable partition and then defaulted to booting from the internal HDD.
