WD My Book not recognized by my new Macbook after being hooked up to my work PC

A year or so ago I backed up my failing G5 using a WD My Book. Two months ago I brought the My Book in to work in order to transfer some files. I can’t remember exactly the message I received but it was something along the lines of it not being formatted for PC and did I wish to reformat. I didn’t want to do that as it would erase the data already on the hard drive so I ejected it and used another hd.

Today, I connected the WD My Book to my new Macbook Pro I received a warning ‘The disk you inserted was not readable by this computer.’

It is showing up in disk utility but in First Aid, the Verify Disk Permissions, Repair Disk Permissions, Verify Disk and Repair Disk buttons are grayed out.

What should I do?

Thanks in advance for your help. 

If your data is save someplace else erase or reformat the drive

if not… look for a data recovery software to retrieve your files and then partition if possible