I chose a 1T external drive for time machine and checked the "encrypt backups" option. I thought it will only encrypt the backups, but as it seems it is now encrypting the whole drive which has quite some data on it and is not SSD ;). Is there a way to cancel the encryption process? I turned off Time Machine and chose another drive for it.
Yes, in 10.9+ you can roll it back while its busy encrypting
diskutil cs revert /Volumes/title_drive -passphrase
You will then be promted with the OS X password unlock/decrypt dialog. After that it starts decrypting. The
title_drive you can look up with
diskutil list or
ls -al /Volumes
Check the status with
diskutil cs list
Look for "Conversion Progress" and check if "Conversion Direction:" is set to "backward"
Then reset pram with option+cmd+p+r.
Booting with cmd+r the utility disc (I have a macintosh HD and one partition encrypted (the one with encoding paused)) >select the partition encrypted>click file and unlock>repair the partition disk>repair permission on partition>repair Macintosh HD. Restart.
The encryption will be resumed.