Oficially you cannot convert DRM .epub files to MOBI.
It is possible to do using a DRM stripper such as epubee.
I do NOT advocate using such code to get around the valid limitations placed on a DRM book.
However, where the problem arises only because of the ineptness of Adobe's ADEPT DRM infrastructure and its implementation,
such a measure is totally justifiable.
I think in your case you have hit a valid restriction and probably should not convert the book.
Unfortunately, Amazon sell Kindles cheap but then intentionally limit their use (no Adobe DRM) in order to lock you into buying books from them,.
this is a library book that I have reserved you would think that if they were lending it out the DRM would be fixed. I am not trying to do anything illegal.
This has nothing to do with Amazon It is the publisher
The DRM is part of the lending: it gives you the right to read the book for a limited period on compatible systems.
In the UK and many other places, libraries use Overdrive which uses the Adobe ADEPT DRM infrastructure.
This is supported by a majority of eReader devices, and there are apps for its use on most tablets and phones.
ADEPT is as close as anything to being the 'open' DRM standard for eBooks.
Amazon the publisher and producer of Kindles has sadly chosen not to support the ADEPT standard on Kindles,
and Kindles are therefore not compatible systems.
I understand that many libraries in the US have also arranged for lending of books in MOBI format.
I am using OverDrive it is used in many USA libraries. It is the Publisher, not Amazon that is doing this. I have used a plug in on calibre which does not work
If you have a suggestion of how to convert this - from ePUB to MOBI, fine, Otherwise, telling me what Amazon does/does not do does not provide a solution.
I use a Kindle, enjoy it and up to know have had no problem with borrowing books in the last 18 months. I have converted ePUB to MOBI numerous times with no issue
As I said in my first post, you can probably only convert it by DRM stripping it first.
Then use Calibre to convert the stripped file in the usual way.