What Causes a failed Console Update (Xbox One)

I call upon the knowledge of those far greater than I, a lowly n00b, in all aspects. I found a listing on ebay for an xbox one x console for a low bid price, but the caveat was the attempts to update it keep failing at 73%. I messaged the seller in question, asked him if he did a soft/factory reset on it to try and fix the update issue, if there were any noises coming from the console that shouldnt, and he says no on all counts, not even a single error code given (not from a lack of effort, just stops at 73% then and there)

I want to know, could the issue with an xbox one console (either base Xbone, Xbox One S, Xbox One X) not updating past a certain point be its internal hard drive suffering a potentially fatal error, needing to send it to whatever repair department its housed to in the mean time?

forgive me, but I thought my first post pointed it out: he can't update this past 73%, whether its online update or the offline method. I asked if the possible reason behind this might be its internal hard drive suffering a failure of sorts

I had a issue similar to this except I could get the update to succeed. except I would get error codes such as E105 and E101. I've tried every method including offline update.. fixed the issue for like a day and it returned.

Today I got the hard drive replaced with an SSD.. did an offline update to avoid having to clone the failing HDD and haven't had an issue so far. hope this helps ..

So to be clear with him, I asked him to expand on all he did to try and get it working, and it seems that after his failed update, whenever he tries to restart it or full boot it (after a proper power cycling), it immediately goes to the "applying update, pls wait" screen and none other. I thought he could boot to his main menu fine, but if what I think is true, methinks his HDD is screwed.

So now if I can beat the bidding war over it, I can get an xbox one x for relatively cheap to repair, think I got a spare hard drive for it around somewhere