The currently maintained version of the DeDRM plugin for calibre can be found at NoDRM's github repository:
https://github.com/noDRM/DeDRM_tools
The version number at time of writing is 10.0.3
Original post:
Spoiler:
Calibre 5.x requires DeDRM 7.1.0 or later
Calibre 4.23 requires DeDRM 6.8.x
DeDRM 7.1.0 mostly works OK. If you come across a book for which it doesn't work, drop back to 4.23 and 6.8.x. Here's a method that lets you run 5.x and 4.23 side-by-side on Windows:
1. Grab an
older version of Calibre Portable.
2. Extract it wherever.
3. Exit your main Calibre beforehand. Open Calibre Portable.
4. Install the DeDRM plugin.
5. Copy your settings (
C:\Users\[User]\AppData\Roaming\calibre\plugins\dedrm.json) over to
[PortableInstallationFolder]\Calibre Settings\plugins\ so that the keys are available.
6. Now there are two options from here.
OPTION ONE (Best for small libraries with few plugins):
a) Open your current library in Calibre Portable.
Only do this on 4.23 as otherwise the databases aren't quite compatible with 5.x. (Note that since the portable version is 32-bit, it
may have difficulty with very large libraries.)
b) DeDRM your books.
c) Close Calibre Portable. Open your main Calibre.
d) Enjoy your DeDRM'd book.
OPTION TWO (Best for larger libraries with multiple plugins):
a) DeDRM your books.
b) Exit Calibre Portable. Open your main Calibre.
c) Use Add Book. Navigate to your Calibre Portable's library and add the eBook file.
d) Enjoy your DeDRM'd book.
Hope this cuts down on the number of posts made about this.