Maybe he is getting "hissy" b/c of your condescending attitude, "eventually you'll come to realize...". Except he already realizes what you are trying to say, he just disagrees that it is best for him.
I have six hard drives in my case. 256GB SSD for Windows and some programs, 1TB WD Black for the programs that won't fit on the SSD, and 4 2TB drives in 2 RAID 1 arrays, these have all my user folders and my data on them. I would eventually like a bigger RAID array in a separate file server but with hard drive prices the way they are now that won't happen for a while. Anyway there are plenty of reasons someone might have several drives. Oh, the dock is a horrible idea, those USB 2 docks are slooooow and many people are only now having USB 3 as an option.
if you are using a program that wont fit on a 256 gig SSD, then you should be using a mainframe instead of a PC but that's beside the point. having lost over 30 TB worth of data throughout the years, i'm simply trying to help him preserve his data integrity. if he choose not to take the advice, he's the one who has to deal with the consequences. how hard do you think it's going to be if he happens to be infected with a particularly nasty strain of virus? loosing drives doesn't hurt anywhere near as much as losing that much data. having drives stored offline is not ideal, however it's also alot safer and actually more convenient since it's alot easier to do a full scan if you ever suspect malware/virus. scanning through 5 drives can take days depending on how slow the drives are, and i remember how slow those 250 gig drives were.... heck it takes 2 hrs just to do a real format.
as far as HDD docking stations, they are all sata/esata these days. nobody really use USB2 for those type of jobs anymore. USB3 maybe ok, but there is too much overhead on the protocol and esata/sata is still faster in reality. for your own data, it would be easier for you to have 2 x 4tb drives. 1 drive for daily use, the other for backup/mirror purposes. alot less headache and alot faster solution since higher data density platter results in less mechanical lag due to reduced read head movement for the same amount of data.
like i said. i can make suggestions, whether you guys follow them is up to you. these are suggestions based on my own personal computer usage patterns over the last 20 years and may not apply to everyone.