The other thing I was going to suggest is use a genuine CF card, not a cheap
imatation as the C80 doesn't like them plus I think it needs to be less than 512mb from memory. Ours is 128mb which worked fine for the 5.16 update.
I have stopped doing any further updates, whilst there is lots of additonal functionality these chart plotters are now quite old and the design and spec dates from the early 2000s. You wouldn't put Windows 10 on 17 year od PC so I don't want to overload the C80 with stuff I am not going to use and risk the whole operating system grinding to a halt.