Locked external hard drive

I have an external hard drive that has decided to lock me out with a message "The folder “Master backup” can’t be opened because you don’t have permission to see its contents."

The drive - a 2TB Toshiba - was working fine right up until my 2011 iMac needed a new graphics card. Since then the iMac and all my other Macs display the same message and entering the (correct) password doesn't get rid of the locked symbol or allow me to access the drive. It does allow me to change the custom access - kinda - but they then all revert back to the previous setting without allowing me access.

My iMac is running 10.6.8 because I need to run some legacy software but the MacBook pro and MacMini are all up to date - none of them seem to be able to get into the drive, in fact the MacBook and MacMini can't even access the disc permissions.

Weirdly, I am able to copy the drive (to a point) but there being 1.5TB on there it eventually errors out and doesn't copy everything over, more specifically my iTunes library which I am trying to acess.

It's a puzzler, I've been using Macs for over 30 years and thIs has me stumped!

JM