The Itch launcher fails to install the game. 

"zip: unsupported compression algorithm"

Windows 10, Itch on primary SSD, game being installed to an external conventional HDD, no other games have had this issue to date

Got it. Thanks for posting. I'm pushing itch's 1gb limit, which lead me to compressing the game in a way which (oops) doesn't function thru the itch launcher.

Good news: all the necessary files are in the zip and it should run on your external.

I'll try a different zip method in the next upload. Thanks again; keep me posted?

Sure.

I should add, it works fine when downloaded and extracted manually.