Back up hardware

You shouldn't backup to a single external hard drive but not for the reason you said.

There's literally zero wrong with going from disk to disk, data on tapes generally lasts longer than on disks but we're talking years or decades not the timescale I imagine you're concerned with.

My concern is that it's a single hard drive you're backing up to, what happens if some data is corrupted or deleted and it takes a few days to spot? at that point you may not have a copy.

It's old-school but I'd use a grandfather/father/son scheme to a bunch of disks.

You should have an offsite backup solution if you really care about the data.

Unfortunately the answer for you depends on your situation and scope of backup project. At my company we are quite large and backups are deemed crucial so we use a Netapp solution.

I would agree with you and shy away from a hard drive to hard drive backup though just over USB. I would consider having a backup server if possible and large storage space so that you can go as far back as needed.

One system a server admin I worked with used was an Apple Xserv RAID. Very easy to implement and not too costly. It scales nicely and since its Java based, can be managed by any OS. So OSX is not needed for the front end management.

As others say, offsite if possbile, but again it depends on your scope.