Not really seeing huge savings with external versus internal. And most HDD's in the external caddies are of lower spec than the desktop options.
6tb desktop drives start at £170 with 2 to 5 year warranties, thats more preferable than buying an external archive/backup drive in a caddy for £180 and voiding the warranty. Though I would rather run 4tb drives anyway.
Like others are saying it depends on the drive, if your unsure, use google as to find out what drive is inside a caddy and it's technical specifications, google may even throw up a video of disassembly. Simply do your homework on Google.
Seagates and Verbatims I know are easy to remove drives from, but unless it's on discount, well the price differences are not usually big enough to risk voiding waranty on.
You also have to do your homework on what the drives are suited to, a quick google shows Seagate 8tb drives as 5900 rpm SMR drives that fail a lot in NAS boxes like Drobo. As a result, I would not spend money on that drive. I would rather save and buy a reputable desktop packaged drive and retain some sort of warranty.
Does your OS support 4tb plus drives?
One of my externals is a supermarket bought Verbatim 1tb drive, it was on sale and cheaper than an internal drive, and contains an,
Hitachi GST Deskstar 7K1000.D HDS721010DLE630 (0F13180) 1TB 7200 RPM 32MB Cache SATA 6.0Gb/s 3.5" Internal Hard Drive.
To be honest, not the most reliable drive from what I have read so it's more an MP3 storage and such non crictical files.
I can remove this drive rather easily, and replace it with as an example my 1.5tb WD Green. The caddy is not the greatest and to be honest I think the drive would be better inside a PC regarding performance. But I bought it as an external drive.
My backup drive is a Seagate portable 4tb drive (2.5" drive) , and again I seen videos showing this being unassembled before purchase. It's a great little back up drive, quiet and quick so great value in my opinion. It will be staying as an external drive.