It is a matter of how you set it up. What i mean by that is if you use the Western Digital software (or Seagate software if it is a Seagate External backup drive) then you will be at the behest of the software and how it functions, IE: if it is "Backup software" than it will most likely just backup your data and that's it, you will be able to restore it if your main drive('s) that are being backed up fail but you "may not be able to access the files at your leisure" as you had mentioned. Now with that said it also depends on how you format the drive and if you are using it as a true "back up" or if you are actually using it as "external storage" for whatever reason, as if you use it for the latter than you will be "manually" coping your files over to the external drive and therefore will have the ability to access them as you wish, however you many get lax in your back up methodology and not backup your data consistently or fully so you must decide it it is 1. A Back up drive, or 2. A external additional storage drive. As for the drives, anyone that has ever taken one or more apart knows that the drives inside are identical to their "internal" drive counterparts, making many of us wonder how come the externals are more often than not less expensive than the internals? as they are in fact the same model numbers and specifications, in most cases.
As for your problem specifically, I would purchase an additional drive, like the 8TB that is on sale, 1.Attach it to your computer, 2. Create a "New Folder" and name it "Back up Software" and then cut and past all software related to the operation of the drives auto backup features, 3.Create another folder called "1TB backup" and then copy, or "Backup" the data from your 1TB drive to that folder on the 8TB (If it not accessable anywhere else, if it is then simply copy and past all files, folders, etc into that folder and once everything is backed up then decide it you are going to keep the 1TB as a additional backup, or format it and create a new 931GB (approx) external storage drive.
Good luck and remember....You can never have enough storage space, i have no less than 11 internal (six 4TB, and five 3TB in RAID as well as two 1TB SSDs) in addition to my two 24TB NAS boxes (4x6TB x2) giving my main system 47TB internal, and 48TB external and that's not even counting the two dozen 2TB, 3TB, 4TB and 5TB external drives so for the price you can't go wrong, $150 bucks for 8TB is a pretty solid deal, even though they had this drive for $130 just 2 days ago, however you could not buy it online (in store only) and apparently for me, no store within 250 miles of me had one in stock, yet today they are magically there....