Before the library is updated, all the books are fully converted/processed and waiting in the temp folder. Same with the database file. So normally either all books are converted/imported or none. Even if calibre suddenly crash, the power is broken, the drive is full or memory runs out during a batch process, the library is still most likely fully intact.
I use a RAM disk for temp, and have even less space, but manage without any problems with 8GB temp storage.
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