https://forums.macrumors.com/threads/1035277/
https://forums.macrumors.com/threads/1036884/
safest raid ? some options
Raid 1/0 or raid 10 same difference but can be done with no hardware card with just the OS so easy and cheap and reliable
raid 5 or 6 is nice but needs a card so external case and card base price $1000 without HDD with some RE3 a about $2000 total so you have to be ready to make the commitment
so without knowing how much storage you need or how large files she works with or any album layouts etc.. ?
going on a starting point which might change with the above info
depending on needs start with raid 1 on some WD 2 TB HDD still will be fast and safe
get a 120 for Boot and OS
WD Black HDD in 2 TB size for backing up the raid 1 to this will only be used incase your main HDD fails ! so a fast HDD is nice
you can partition this with a top partition of 200 gigs and use this for a scratch disc for PS
and the rest for your clone BU
also remember you dont want to fill up a HDD you work on over %60 better at %50
so that bu will have plenty room left over
get a nice external case something like a 4 bay case from OWC the mercury use this for time machine
this is used to get files you need back think Human error here
if you get one filled with large HDD like the 2 TB you can run it in raid 5 and get 6 TB of storage you could say use 3TB for time machine and the other 3 for archiving work ? again lots of options here
if she works in LR
get a SSD for the cache and second use as scratch for PS
if you go this route use the SSD as first scratch and the 200 gig partition for the second !
so
bay 1) boot SSD
bay 2 & 3) your raid 1 storage
bay 4) your clone BU
the only thing missing in this scenario that is a must is a clone of your boot drive !
you could put one in the extra optical bay or use that for a SSD scratch
we can fit the boot clone in

but need to know does she work in LR
how big files ? how much storage
this should be a good start
I work as a Photographer and also have a post production studio that does album design and PS work for pro photographers

so do quite a bit of work in PS and LR