I have found out about the problem, I had the same problem with my pi 3645, some times it wants to boot and most times not, it wants to not read the operating system, after a whole lot of experiments I thought that it's Seagate 1 Terabyt (TB) the hdd that is something crazy with and a thing more, it was for repair news got a hdd in (of course the same seagate 1t hdd) I bought new western digital hdd 500 gb
installed the operating system on it, and since then I have never had boot problem
and the last thing i will write here, the 1 tb and 1.5 tb seagate have freezing and boot problem, there are a new firmware updates to the bad firmware seagate hdd.
sorry for my bad english
the best regards and luck