Probably best to hear from JMBach on this but I think the KS58s are just a precaution. You could probably do the whole procedure without them, I have. Since this is a Roamio as long as you keep the original drive intact can start over anytime. Yeah, the initial copy can take awhile. I run 58 occasionally if I am having any issues, can clean up some problems that have developed over time.I'm not freaking out yet, but I am a little concerned that my TE3 Roamio Pro doesn't seem to be able to complete a KS 58.
I did the 3TB-->8TB mfscopy to create the initial 4TB image which booted fine and successfully connected to TiVo network service. Recording capacity went up to 699 HD hours and My Shows reports 63% full vs 91% previously. Cablecard/TA settings are still there, The to-do's, guide info and icons all look fine.
I followed what I think is the recommendation and did a KS 58 before proceeding on to the first mfsadd+fix expansion . I let it run for more that 3 hours on the "Installing an update. This will take a few minutes" screen. Since @jmbach said KS 58 shouldn't take more than 3 hours I pulled the plug at about 3 hours and 20 minutes just to see if I was hosed.
I re-applied power and again it booted-up fine and successfully connected to service. I even let it record a scheduled show. Then I tried another KS 58 and I've already waited even longer than last time, 4-1/2 hours so far, still stuck on the Installing an update screen.
I'm wondering if I should just go ahead and do the two expansions to utilize the full 8TB and not worry that it won't do a KS 58.