Backblaze essentially 'mirrors' your computer, minus OS files and programs. When you remove a file or a drive is disconnected, Backblaze removes those files from the current backup set and will delete them in 30 days.
There is one workaround to prevent the 30 day deletion, however, this pauses your backup and you can not backup until all of the drives are connected once more.
You can go to the Backblaze Control Panel (Windows), or the Backblaze Preferences (Mac), click 'settings, click 'schedule' and switch the schedule to 'only when I click', then make sure the backup is paused.
This won't technically stop the countdown, but it will stop the client from submitting the timestamp that will notify the servers to delete the files. You will want to make sure that you have the drive plugged in or restored all the files before resuming the backup, otherwise the files will be deleted.