External HDD password is not accepted

Hi and thanks for taking the time to read my question.

I have an iMac 3.06 GHz Intel Core 2 Duo running OS X Yosemite, I use a 3 TB WD External drive to store both my iTunes and Photos libraries as both are into the hundreds of GB's, I normally leave my mac on, though sleeping and restart it every couple of weeks, I have several accounts on this mac that family members (Brother, Niece and Nephew, Girlfriend) all use their own, my external drive would appear for them, I'm not tech savvy enough to prevent them from 'seeing' the drive when they log in so I put a password on the HDD.

My issue is that either Mac OS X or the HDD is NOT accepting the password.

I have quite a clear password hint and this isn't some obscure password, it is long but I use it elsewhere on my mac and I can access those disks / log in's and I doubt I would have written it incorrectly twice when choosing it, I recall having this issue before but eventually it just worked, I've tried over a dozen times now though.

Is there anyway around this? I absolutely cannot erase the disk, I would lose over a decade's of irreplaceable photos; iTunes I know i can replace, the photos I cannot.

If you're only answer for me is 'tough luck, erase the disk' thank you but I don't need that helpful hint haha

Thanks again for taking the time to read this lengthy question, any help is greatly appreciated.

EDIT: I've repaired the disk and it worked! thank god haha not sure why I didn't try that before.

Message was edited by: MattCasey