I have a USB drive that's used to backup my 'live data'. I have heard of malware that will encrypt all attached drive so I would I'd like to give the the backup drive some extra protection. Encryption would slow down the backups so I'd like to simply have a password to protect against this type of malware.
I could simple take the drive off line but I think password protection would work.
One backup is not enough - - you need a second one containing another copy of those backups and stored safely away - - I keep three copies of my data on three separate external HDDs. If malware strikes any one of them I just reformat it and restore the data from one of the others.
No good relying on a password to stop malware accessing a drive anyway -- it won't stop it at all. Just don't have any backup drives connected to your PC until you actually need to use it. Then disconnect it immediately when finished with it.