Adding External Hard Drive

I currently have 5 USB hard drives and 1 Blu-ray drive connected to my Mac mini via USB hub. No problems - the only issue is that if you want to boot from an external drive, it must be connected directly to the machine, not through a hub. That's usually not a problem since I boot from the internal drive.

I plan to drop my new iMac in place of the Mac mini when it arrives next week, connecting all external drives the same way. I don't expect any issues.

Right now, Seagate external drives tend to be cheapest per TB. I have 3 of their "Expansion" line.

I like Western Digital internal drives (combined with separate enclosures) for reliability, but I can't recommend their external drives. This is because they apply some sort of proprietary encryption to the drives: if the USB controller in the drive fails, you can't open the enclosure and pull out the bare drive to recover your data.

My best recommendation is to have at least one Time Machine backup drive as you mention, and maybe a second backup as well (either another TM or just manually cloned). Drives from all manufacturers fail, often with no warning. Better to have extra copies of important data than no copies. I recently had 3 different Seagate internal drives fail - all from the same batch, all failed at 2-3 years old and out of warranty. My 3 Seagate externals are doing much better, different batch.

Hope this helps.

 

For my Time Machine backup disc I use a NAS:

Synology DS212j with two mirrored Western Digital 1TB HDDs (5-year warranty).

And I am very happy with it.

 

Although this doesn't meet your requirements, it's my favorite drive as it is small, super fast, and large. Perhaps worth considering? Although it is RAID, it is invisible. http://www.seagate.com/external-hard-drives/portable-hard-drives/performance/backup-plus-fast-hdd/