I have a WD MyPassport 2TB drive in an external USB 3.0 enclosure that I use for backup purposes.
Every few months, I manually copy my entire Documents and Music folders to the external drive.
There's about 1.2TB on it now.
Problem is, I plugged it in for the first time in a while and accessing data on it is EXTREMELY slow. Like, to get a directly listing in Finder takes about a full minute. To copy of directory with 1GB from it will take close to an hour.
The drive appears to be working, but it's running extremely slowly.
I may have done an Mac OS upgrade or update since the last time I used the drive ... I'm currently running macOS High Sierra version 10.13.6