My Book not accessible on iMac

Faithful mid-2007 iMac - the first Intel model- (El Capital 10.11.6) entered sleep mode and would not return or reboot beyond opening screen until I removed the firewire WD My Book (of same vintage) used for Time Machine. When the drive is connected the System report shows WD is connected to firewire port but it will not show up on desktop. Disk utility does not see it and Time Machine acknowledges but cannot open the drive. The drive shows up normally on my MacBook Pro (2009) and the file structure can be revealed so I conclude it is not the drive or connection.
Diagnostic info points to a software issue. I dont remember any updates in the last two weeks that may explain this.
iMac behaves normally without the firewire drive.
Any ideas? Any recent firmware updates that would explain this?

Unfortunately, no firmware updates are available since there are no FireWire units in WD’s current product family. Does your unit work on the same system over USB?

Thanks for the response. The My Book unit has rwo FireWire inputs and no USB so I couldn’t try that. It’s an oldie.

I notice now that the iMac freezes ( browsers become unresponsive )when the drive is connected and powered up. IMac returns to normal when rebooted without the drive. Strangely, my 2009 MacBook Pro accesses the drive and I can drill down a couple layers in the file structure normally.