I have several years of FCP projects on FW 800 drives, and have moved some of those projects and media to Thunderbolt drives using 7toX, when needed.
I'm archiving my raw footage to Blu-Ray discs and am editing FCPX projects using 2 4TB Thunderbolt drives, each configured in RAID 1, so I have 2 media drives, each holding 2TB. I'm using a 6TB Thunderbolt drive, configured in RAID 0, as a daily Time Machine backup of the 2 media drives.
My 6TB backup drive is filling up and I'd like to ask for suggestions on how to move forward with backup and archiving.
Well, it's not exactly an answer for you, but here's my workflow that you might take some portion of:
All media, projects, etc are ingested into a RAID 5 work drive. Nightly backups (altered data only) onto a single cheap, slow external drive. (hey, I'm not watching it... and this backup is just paranoia on top of the RAID 5 anyway.) Archival onto a RAID 1, so when full, 2 bare drives go onto shelf. External bare drive mount device for accessing archived drives. If any archived drive has issues, it's mirror is cloned onto new drive to maintain 2 drive set.
All drives are regularly scanned with Disktracker, simple, fast database app. Makes finding old projects very fast.
I use a more paranoid archive and storage setup.
Time machine backups system and FCP and Motion projects, (no media files but graphics or text I put into the project folder, no render files) onto an external FW hard disk box, on daily basis. When it's almost full, I remove the HD, slip it into a cardboard box and into a drawer, and replace it with a new one (I don't let TM erase old backups and build new ones with disk almost full, and I don't trust external enclosures for heavy and long use of HD's). I have a RAID 5 6TB NAS I use to store a copy or the raw footage, DMG's of the cards named with date and place, "media managed" projects and HD QT files of each project I have delivered, complete with music bulletins, scripts, documents, licences and everything that goes with it. Then I have some more disks I store a redundant copy of the raw clips and DMG's upon, numbered and dated progressively. One 1,5 TB can usually store one year of work. Older footage that does not need to stay on the RAID 5 array gets a second copy on one of the older Time machine disks that are no longer needed and lets free space on the NAS. So whatever disk or storage decides to abandon me, I can rebuild everything I need, present and past. For editing, I have four disks inside my Mac Pro, one for system and apps, one for scratch and export, and two in RAID 0 for the media used in current projects. I am not deep into FCPX yet, but it should not be much different ( I don't think I would backup render files or whatever the app can rebuild starting from the original raw clips.