Best Airport Time Capsule HDD?

For single drive setup of the TC either will work pretty well identically.

The RED is designed for use in NAS with multiple drives. Since this is NOT the situation with the TC it does not really matter.. I run a Purple in my TC. They are a little slower I think to keep the noise level down. Buy what is cheapest or more readily available. The better end 8TB drives are expensive. And you do not want Enterprise drives because their power consumption / heat exceeds the TC ability.

** I did a quick survey at what is going at the moment and the Seagate Ironwolf series is probably pretty much the same as the RED. There is also somewhat more expensive HGST/Toshiba models which generally have good reputation.

Everyone has bad experiences which is one reason you buy one brand for the TC and a different brand for the backup drive. At least they are less likely to die on the same day.

If you are going to store lots of your files on the TC.. please do not forget to backup the TC to another drive. Never forget files must be on at least 2 drives.. and backing up the TC (with even a half full 8TB) is going to be total pain.

Time Machine cannot backup network drives. If you use something like Carbon Copy Cloner.. it can do the job.. but you should use ethernet connection to the computer and another 8TB drive plugged into same computer by USB3 or faster interface.. or in the network.. not plugged by USB to the TC which is super slow.

** yet another NB..

As a storage device.. the TC is pretty dismal.. From a Mac or a PC the same disk in even a half decent single or dual disk NAS will do twice the read and write speed. Transfer speed from TC is processor bound.. using faster disks doesn't do anything.. and they are really not designed for NAS work.

Assuming you have the AC model TC.. take great care removing the existing drive. The ribbon cable is fragile.. and you must remove it from the back of the drive.. very gently.. don't try and release it from the board.. the ifixit instructions are not clear.

Also the new drive will fit but the old rubber mounting blocks will not.. even cutting them up is really hard.. the existing seagate drive is only two platters and extremely thin at the end.. the 8TB will be full size so you will need to be creative for mounting.. but it is good to isolate the disk from the metal chassis so you don't get noisy vibrations.. strips of rubber high density foam should work. On my desk I have the TC about 1M from the keyboard.. and I can feel the vibrations of the drive running rather than hear it. So I do know when the computer is doing a backup.

None of that applies to the earlier model TC.. which are much more ruggedly built.

As for backups.. I didn't think you would have internal drives of 8TB so was concerned you were dumping files to the TC to save space. Once you remove a file from the computer.. Time Machine will delete it from the backup.. eventually.. and besides if it is stored on the same disk as the backup.. that means a single disk failure takes out both original and backup.. You seem to be aware of things.. but I always make the warning as people forget.. and losing 8TB of files can cause much crying, gnashing of teeth and shouts of despair.. !! People with large NAS often think having RAID will protect their files.. until the ATOM chip in the NAS dies for example.. as has been happening recently due to BAD BAD Intel mistake. Raid files are extremely difficult to recover and all that was needed was to use the NAS inbuilt backup to USB drive and all would have been fine.