JamesReardon,
External drives are fine. Jim Simon may use internal, RAID5 100% of the time, But the rest of us frail human editors use a regular mix of internal and external drives, RAID0 - RAID5, or no RAID at all without issue. Both USB 3.0 and Thunderbolt are capable of transferring a whole lot more data per second than ANY hard drive (or even many SSDs) are capable of. The only caveat to this is if you're using a SLOW drive to begin with. But in any case, Hard drive X in external form will perform just as well as the same hard drive internally. To Jim's point, however, internal drives are certainly less expensive than external drives. But more to the point, I'd be very surprised if your external drive situation was the cause of your "offline" clips. Wish I could help. Carry on.