This is not a great setup unless there is also a backup function being performed.
If you want redundancy (one unit can go down and the other still functions) this is fine. If you want protection against file corruption, you need to get a copy of the file onto another unit, that will not be touched after it has been copied. Mirrored drives just make a mirror, so if you corrupt a file on one, it will be corrupted on the other drive as well.
Absolutely. RAID is almost worse-than-nothing (in that it provides a false sense of security) regarding backup for the use-case the OP describes. Of all the data-loss I've seen our customers suffer the least common is a hard drive crash.
Here are a few of the many ways you can lose data which an internal RAID will not help you with:
User error (not always the photographer, could be the tech/assistant/etc) resulting in deleted, overwritten, misplaced, formatted, or otherwise mishandled.
Software error like the infamous OSX update which changed the way file-caching was handled resulting in, for a brief, but horrifying period before everyone patched their software to accommodate the change, where some software and camera combinations would seem to be recording the images but if the software crashed there were no actual files on the hard drive, only in an ethereal cache which evaporated when the software crashed
"Kaplunk" meaning the laptop/drive/camera took a swim, was pulled off where it was sitting, or otherwise suffered catastrophic damage
Moveover most hard drive failures do not happen spontaneously but are preceded by some period of clicking, deteriorated performance, or won't-boot-first-time indications.
The chance of a hard drive instantly failing in the middle of a shoot is extraordinarily unlikely.
Incremental, user-prompted, frequent, physically isolated backups are IMO the most steadfast way to protect against the myriad sources of loss. They also happen to be extraordinarily easy for almost all use cases. Super Duper, Carbon Copy Cloner, or any of the other backup software and an external drive (or two with one being rotated out and placed outside of the action-area) takes little thought, little money, and is very easy to use.
The only use case I would suggest RAID for is when a loss of the last 3-5 minutes of a shoot would be of enormous consequence*. This is usually, even for high-end high-pressure shoots, not the case and the emphasis should be on protecting against the (far more likely) loss of the last several hours of shooting.
*Since user-prompted backups don't occur in real time (which would be a performance hit) the period between the last user-prompted backup and the next user-prompted backup is the only window in which data could be lost. In every shoot I've been on there has been a period of rest every 3-5 minutes sufficient for a single keystroke to start a backup followed by knowing computer operation might be a hair slower for 10-30 seconds.
