With any new deck that has multiple features and multiple ways of doing various functions there is a learning curve. I find all my problems are user error, but I try to experiment all the time with menus and features as this teaches me and allows me to get comfortable with each deck. (Unfortunately I am at an age though where it is easy to forget when I do not use a deck for a while
)
The up side is that I do get very comfortable with using each deck and it's features and I find the way that I like best for settings and how I will use the deck. I have not had any problems with 4.01 firmware nor any of the earlier firmwares and I think 4.01 is way better than any of the earlier firmwares, but everyone should do what they like best. I usually record 6 mics on my MixPre6 and now MixPre6II, but I record at 24/48 and now 32/48 and that sounds great to me. I mostly use Sony and Fuji 128GB cards and a few 64 gb San Disc too and have never had an issue.
There is always a risk that a new firmware will have a glitch though, but usually I am willing, sometrimes I might wait a week or two and listen to what if any complaints arise.
Regarding the USB backup feature...In my opinion this may be good for a film person or a person working long hours in the field, but for me recording shows, it is not useful. The backup is slow, it takes a very long time to copy a 6 mic 75 minute set which at 32 bit float with sampling at 48kHz is often 2 files, a great deal of data. There is no way it will copy the entire first set in the break between sets and of course no chance for copying the 2nd set before I break down (or multiple bands with even more sets).
I have never had a card failure in, so the backup would be nice, but not anything I need. It turns out that it is much easier to go home throw the card onto my PC and back it up that way. It takes a few minutes and flies through the data transfer.
I have not experienced a lockup or inability to turn off my deck while it was attempting to transfer, but I did get a 0 kb file until I asked SD about it and got their instructions and tried the backup at home. Like I said it is slow, so I will not use it, but I think it very likely that many of the issues and lockups could be from having the copy feature enabled. It is just one example of where not fully understanding the deck or the settings chosen can seriously affect the outcome.
That is one of the reasons we have these threads, so we can all learn form each others mistakes.