Originally Posted by kevinofengland
The only reason I wanted to delete them is because when I sync the new Kindle I only want mobi files to be synced. Will Calibre send every file or only compatible ones? If it is the latter I won't bother deleting.
For each device type you can configure the formats that you want to be allowed to be sent, and put them into priority order (if more than one compatible format is found). If you try and send a book for which there is not yet a compatible format Calibre will ask if you want it to be converted as part of the transfer process. You should look at the Kindle device plugin for the appropriate settings in your case.