My external hard drive is locked and I can’t get it to unlock. Help!

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This can be a big problem. I’ve seen it happen for a lot of reasons. Windows can change the permissions of a drive along with Mac OS X. As long as the drive can mount you can try the following.

In terminal use the following commands keeping the quotations

sudo chflags nouchg /Volumes/"TM drive name"

sudo chmod 775 /Volumes/"TM drive name"

Special thanks to V.K. in the Apple support forums here: Apple - Support - Discussions - Time Machine Backup Drive Locked ...