Just to add to and reinforce a couple of the comments above:
1. If you are backing up to just a single disk there's no point in the extra expense of lightning. You will be limited by the speed of the disk. Running Blackmagic tests on a couple of my disks showed 42.8 MB/s write, 42.9 read on a FreeAgent drive and 75.6/82.5 on a backup plus drive. So for the first disk even USB2 might work (60 MB/s theoretical). For the faster disk USB3 would be overkill - 640 MB/s - almost 7.8 times faster than the drive (theoretical).
2. For How long do your want to keep your Time Machine backups? The larger the drive the longer that you can keep your backups until Time Machine starts deleting old ones. One of my two TM backups goes back to October. 3.17 TB is backed up, and 1.29 TB is still available on that 6 TB disk. For my largest directories (Pictures, Music, Video) there isn't much churn so I don't have lots of versions of very large files which would fill the disk very quickly.
My personal experience is, however, that the longer the TM disk has been used the greater the chance of an error. I have had to rebuild this disk with Disk Warrior multiple times. But this is likely due to my setup.
Again I have had the unfortunate experience of trying to restore from a TM disk and the restore failed as the TM image was corrupted. I haven't found any reliable way of verifying that a TM image is good.
3. Somewhere I remember reading (think it was Larry Jordan's website) that if you have something critical that you absolutely do not want to lose you need to keep it on 3 different devices on 3 different media types. For example backup to disk, tape, and online. If you are just using disks then they should all be different models, ideally from different manufacturers. His perspective is for a professional video production site, but I think the advice is valid.
4. Don't backup to a disk and expect that disk to be useable in 5 years, for a number of reasons.
a. If file formats or drivers change you might not be able to read the drive.
b. Connectors change. I forgot that I had the BlackMagic numbers above in Evernote so I was going to hook the drives up to get the numbers. One of the drives is FireWire. I had cleaned up my cables and can't find the Firewire cable. And even if I did find it I would have to use an OWC dock that gives me a Firewire port since my computer doesn't have one. At some point OWC and other vendors will no longer make lightning to Firewire docks and these disk will just be paperweights.
b. Drives have a half life. The magnetic information on the disk fades over time if it is not used. This is done automatically if you are using the drive, but it doesn't happen if it is in storage. A recommended procedure is to read all of the files on the disk every 6 months. You can do this using the unix command at the terminal
sudo cat /dev/rdisk0> /dev/null
Here are links to the original articles:
https://larryjordan.com/articles/hard-disk-warning/
https://larryjordan.com/articles/technique-refreshing-hard-disk-storage/
Again, this is advice for a professional video shop but it's something to keep in mind. If you ever need online video training for FCP or Premiere Larry is the guy you want.
5. You can get unlimited online storage for $60 a year. This is Crashplan's price, but there are at least two other vendors with pricing ~$100. I backup about 8.2 TB (unlimited) for $60 a year. Dropbox charges $99 a year for just 1 TB.
