HB is correct about needing AutoChart Pro to do Si Mosaics. I have the Helix12si gen1 and AutoChart pro. I have been running the AutoChart Live option on my Helix as well as regular Sonar recording. I can see the AutoChart Live running on the screen and the files are stored in Memory on the Helix (unless I have my Zline card installed in which case all AutoChart Live files transfer to the ZLine card).
When I try and upload the AutoChart Live .ACU files into AutoChart Pro for editing they won't load. I get an error message about an incorrect format. HB technical support so far has not solved my problem.
Honestly it is more of an inconvenience/aggravation than anything else since the regular Sonar recording .dat files contains exactly the same information PLUS the SI data files. AutoChart Pro accepts these .dat files so I edit them and build my maps including SI Mosaics if I choose.
I believe HB will work out the AutoChart Live upload issue otherwise why even offer AutoChart Live as an option on the Helix if you can't actually use the feature.
Incidently you can usually pick up a new copy of AutoChart Pro on Ebay for around $175 including shipping.