Hi,
I had most of the books on my device and recovered my library structure including additional columnns by importing them into Calibre - plus 2,5 hrs of handwork and finetuning...
My new library disaster prevention strategy

is the following:
- Synchronizing ebooks and library between my local drive and my NAS
- Backup of the NAS every two weeks on an external harddrive
- NAS is stored externally in my office safe
Thanks for your help, great forum!