Afternoon trading May 30, page-372

O/T -
"......a new back up disk" -
My suggestion is make sure you buy a big one. Mine is a Toshiba 934gb disk purchased for $129 in May 2014. It is still only 1/3rd full.
I was in your shoes (about needing a bigger back up disk) 7 years ago discovering after the event, that after I'd done previous regular back ups - on a much smaller external disk - that when I wanted to recover the data not all of it was on that smaller disk, because it - as they still do today? - backs up your Windows OS first every time, and then in what ever space is left over is filled by data from your device.

How it selects that data is way beyond my knowledge suffice to say that
for each of my email Inbox folders (350) and Sent folders (150)
those that contained 25 emails or less were always downloaded in full;
but for those that contained more than 25, the # downloaded process seemed to be done pro rata, chronologically by date, for each folder!
resulting in 2 to 3 years emails (out of 7 from 2014 back to 2007) for some personal folders containing family pics) not being backed up at all!

i.e. take advice from your retailer which I'm sure will include them telling you that progressively over many years your back up disk space is eroded by the backup of the continuing increase in the downloaded data per your Windows System's automatic upgrades by Microsoft.
i.e. for your data you've backed up from jpg files, spreadsheets, websites, emails, in terms of the minutes backup time, is a doddle, compared to what Windows System data takes.

To put that into perspective 8 years ago (When I had WIndows 7 installed) I would automatically schedule 7pm on a Sunday to do my backup, which then took about 20 minutes.
Now it takes 50-60 minutes or more. LOL with your choice.