[unboggling]

To expand on what I said earlier: excepting some PDFs, I keep only EPUBS even though I read mostly MOBI sent to Kindle "on the fly". I consider these EPUBs, once cleaned up, the master copies. I've rarely had problems with original MOBI conversion to EPUB "master copy" then a later conversion "on the fly" back to MOBI again. In the few cases when I tried to read the result on Kindle and found problems not previously evident or fixed after my initial assessment, it wasn't difficult or time-consuming to fix those specific problems to make a better EPUB "master copy" then successfully convert that "on the fly" to MOBI.

There are other reasons besides those I mentioned in the prior post (KISS, limit confusion, EPUB fixability). One is to speed up performance. I use a custom #formats column because it is useful, particularly during initial assessment after adding, to show the formats in booklist rather than looking for them sequentially by title on the Book Details panel or Edit Metadata form. I suspect that if I had 3 or 4 formats per book title in a large library, calibre would need more time to startup initially or accomplish any other operation where formats needed to be iterated through. As a side benefit, minimizing the number of formats per title also reduces time and space needed for backups and library copies.

Yes disk space is cheap, but I also have backups. The calibre library multiplies itself. One automatic hourly backup to one disk. One automatic daily backup of a separate disk that holds a (weekly) backup copy of the calibre library folders. Original library, backupOfDisk1. copy of original, backupOfDisk2. So for 3 formats per title it's not just 3 times the space to accommodate those extra formats. it's 3 formats x 4 disk instances, complicated further by factoring in calibre and processor time per frequency of relevant operation.

I understand many experienced calibre users feel strongly about keeping the "Original Incoming Format." After a brief assessment at arrival into calibre and occasional cleanup where needed, I've found that keeping "Original Incoming Format" isn't necessary, and dealing with the few exceptions per 1000 EPUB "master copies" is worth the trade-off. The time and hassle saved outweigh perhaps having to buy a few extra books per year to replace any that I've previously made unfixably serious mistakes with -- a last resort I haven't needed once during 12+ months doing this basic workflow and reading this workflow's products.

That it works well for me doesn't mean it will work well for other people. There are many factors: Choices of original sources and material, which vary in format quality and format complexity. Skill at assessment and cleanup when necessary after adding books. Time available for doing initial assessment and cleanup. Type of reading material (in my case mostly fiction with few graphics and minimal complex formatting). And personal and subjective things such as tolerance-level and annoyance-level for relatively minor imperfections in the formats.