I tried to install Debian Stretch on an USB drive.
Installation of grub2 failed.
The USB drive uses
1f75:0888 Innostor Technology Corporation IS888 SATA Storage Controller
with Crucial BX100 SSD.
usb-list: Bus 02 Device 02: IS888 USB bridge [1f75:0888]
usb-list: Level 01 Parent 01 Port 01 Class 00(>ifc ) Subclass 00
Protocol 00
usb-list: Manufacturer: Innostor
usb-list: Interface 00: Class 08(mstor) Subclass 06 Protocol 50
Driver usb-storage
/boot partition ext2.
/root partition ext4 on LVM on encrypted volume
See appendices
Best regards
Heinrich Schuchardt