Got over 50TB of downloaded stuff in external hds (with probably 40TB actually worth), so at this level I can't backup my storage. So, I basically backup the stuff I download there (sometimes after watching, sometimes without). I usually fill 1TB of data of 48G-related variety in a year, so once a year I'm forced to buy a bigger HD, move the information from the old to the new one, and use the old one to backup something else. So, I have a good rotation of disks (for example, my 48G variety one is now a 5TB disk, once it's getting full I will get a 6TB one, copy the information there, and then move stuff from a 4TB disk that's nearly full to that 5TB one).
Of course, a few disks got errors (my old 1080p LOD hd failed, and so one of the files got 5mb corrupted and never recovered them). My Drama HD got a couple of errors too, but haven't bothered to check which episodes (if any) were lost. And my old 3TB anime HD completely failed, sometimes it can be recognized by the computer, sometimes it can't, sometimes I can copy a file to the computer, sometimes it doesn't let me at all. I just hope that, by the time the HD crashes (and all of them do) to no longer care about the contents (like with the anime or drama disks).
- AKB Variety (5TB, main shows, radio, streaming)
- AKB Variety 2 (1TB, members as invitees or segments in non-48G related programs)
- AKB Dramas (1TB, AKB-related dramas)
- 48G Concerts (2TB, 48G-related concerts as they aired on TV)
- AKB48 ISO (1TB, ISOs for singles, albums, members' singles and subgroups within AKB48)
- AKB48 ISO 2 (3TB, ISOs for varieties, movies, photobooks, etc)
- AKB48 ISO Concerts 1 (4TB, ISOs for concerts, 2008-Sep 2014)
- AKB48 ISO Concerts 2 (4TB, ISOs for concerts, Sep 2014-Now)
- SHIMAI ISO (4TB, ISOs for singles, albums, varieties, movies, concerts, etc from AKB48's sister groups only)
- MOMOCLO ISO (4TB, ISOs for singles, albums, varieties, movies, concerts, etc from Momoiro Clover)
- LOD 2008-2012 (4TB, 48G daily theater performances from 2008 to 2012)
- LOD 2013 (3TB, 48G daily theater performances during 2013)
- LOD 2014 (4TB, 48G daily theater performances during 2014)
- LOD 2015 (4TB, 48G daily theater performances during 2015)
- LOD 2016 (5TB, 48G daily theater performances during 2016, includes some from 2015 that didn't fit previous disk, will fill this one by the end of the year)
- Varieties (2TB, no AKB-related, Momoclo or varieties in general)
- Idoling!!! (1TB, Idoling!!!-related stuff including episodes and ISO of live concerts)
- Concerts (1TB, non 48G-related concerts as streamed on TV)
- Dramas, Series, Movies (2TB, non AKB-related dramas, including western series, this one got maybe a dozen bad blocks)
- Anime (3TB, unreliable)
- LOD 1080p (2TB, LODs in 1080p, already copied those to the respective disks, keeping this one just as unreliable backup)
- Miscellaneous (1TB, disk with some old stuff, old ISOs from groups I used to follow like ELT, Do As Infinity, also software and some movies).
So, 50TB of pure idol-related data i actually care about. I check every disk at the beginning of the year, usually between February and March (except the ones that I buy during January).