Problems in OS X with Transmit “Mount to Disk” after Mac upgrade to El Capitan
This was/is a known issue on transmit 4.4.8 and lower. 

Go get version 4.4.9 or later to work around the known issues.

The developer support page here should be updated and the second link is the announcement of the delay:


https://library.panic.com/general/mac/el-capitan/
https://twitter.com/panic/status/648970332473192448



  Quick El Cap Status: Coda mostly works (2.5.12 will bring fixes) — Transmit works — Transmit Disk doesn&#39;t work, but we&#39;re working on it. &mdash; Panic Inc (@panic) September 29, 2015 


You can check for updates or get on the developer's email list / watch their blog and twitter account for when they have released a version of Transmit Disk that works with El Capitan. The rest of the normal functionality seems to work for me, just the disk mounting part is needing work.