Are there any audible ticking noises?
Sadly drives die and the old adage is to back up yet even with 30+ years experience building and repairing Pc's and qualifications in electronics and IT I still fail to back anything up and just go into rant mode every time I lose a drive.
Portable drives are a great idea but you need to protect them against drops, even slight bumps when using them can cause the heads to go out of alignment.
I'm not familiar with Spinrite so I can't make reference to it but most manufacturers have their own diagnostic software which if you can get the drive recognised could be of assistance.
https://support-en.wd.com/app/answer...jE%3D#subject3
Not sure if the WD link may help but the Easeus link has some good information:
https://www.easeus.com/computer-inst...t-working.html
One mention, if you're plugging the drive in via any form of USB hub it may not have enough power to spin the drive up properly if it's not a powered hub and I've known a lot of people to have that problem.
Also, if its USB3 compatible I believe the USB 3 ports may have better power regulation than the older USB 2 ports.
Drives are power hungry and some USB ports utilise a power share system where there's a maximum of X power available to active ports. you could always try unplugging other items which may be connected to the USB ports and see if that makes any difference. Though trying another computer somewhat rules this out.
USB is one thing that despite standards in the industry there's often discrepancies in the products.
https://en.wikipedia.org/wiki/USB#USB_Power_Delivery