My 2TB external drive keeps disappearing every time I boot my Mac
After upgrading to High Sierra, I had the same issue with my LaCie shared not listed in shared devices, coupled with SMB hanging without connecting to remote servers.

The SMB issue was solved by changing the SMB version to v2 in the /etc/nsmb.conf configuration file and rebooting the Mac as recommended in this post on SMB share deadlock. After the reboot with the new /etc/nsmb.conf, shared devices were again correctly listed in the sidebar including my LaCie shared disk, so I surmise both issues are linked.

The /etc/nsmb.conf file I use is:

$ cat /etc/nsmb.conf
streams=yes
soft=yes
signing_required=no
protocol_vers_map=2
file_ids_off=yes