Dear Mosu_D,
From what you have described, it sounds like you are OK with the fact that any data you had previously on the disk is lost, and you just want to be able to use it again. I understand, it can be frustrating to try and clear out an encrypted filesystem. It does sound like your new system is mounting the volume as read only, or having trouble mounting the volume at all... There are two things that I would like you to check out and attempt...
1.) Take a look at the physical SD card. Sometimes these cards have a little switch, which will enable read-only mode. Please see the attached screenshot for an example, and make sure that this switch has not enabled read-only mode.
2.) If the disk is not set to read only mode, please attempt to reformat it again. This time, make sure you uncheck the box for "Perform quick format". A full format is required to remove all traces of the previous encrypted filesystem. (example screenshot below)
Please let me know if this suggestion resolves the issue, or if you require additional assistance or clarification. I'll be happy to help.
Best Regards,
Phil