"some contents unreadable" on HDD

Hi Group!

Hope you can help me out here.

I have 3 HDD's and an SSD installed, that can be selectively powered-on by push-button switches - no sense spinning HDD's when they're not needed.

My Linux Mint Cinnamon 19.3 is installed on a 1TB WD Black, and works well.

A week ago, I wanted to transfer some files from the WD drive to the 1TB Toshiba drive, so I powered-it up also before booting to Linux on the WD drive.

Booted O.K. (as usual)

But, when I went to look at the Toshiba drive in "properties", it said "Some contents unreadable" - 50GB roughly.

I wrote a short file to it, and can read it as well.

Any ideas on how to access these 50GB of my files?

From Toshiba Properties:

Name: Toshiba

Type: Folder (inode/directory)

Contents: 1 item (and 1 hidden), with size 172.7 kB
(some contents unreadable)

Location: /media/ralph

Volume: Toshiba

Free Space: 933.3 GB
50.1 GB used
933.3 GB free

Total Capacity: 983.4 GB

Filesystem type: EXT3/EXT4

I can read & write to/from the drive, but can't reach the 50 GB of my previously-written files.

I really don't understand what has gone wrong here.
Any suggestions would be appreciated.

Thanks in advance,

- Ralph