How do I re-encrypt my time machine backup disc

I've just moved to the new iMac, and purchased a new external drive to continue time machine backups. It was encrypted with a password, however my system seems to be constantly ejecting the disc and I am asked multiple times a day for the disc's password. So I removed the encryption to stop this, but now time machine won't back up on that disc, because it was previously encrypted. Can I fix this? I can't find anywhere where I can re-encrypt. It cost a fair bit so I hope I can do something. And can I stop my iMac ejecting my disc, so I don't have to regularly put in my password? Thanks