Backup with new WD MyBook extremely slow

WD External Drives External Drives for Mac
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Brand new 8TB MyBook, repartitioned into 4 partitions. The one I’m using is formatted as MacOS Extended Journaled.

I have it on a Catalina 27" late 2012 iMac, it’s on a USB3.0 port per the Hardware Summary.

A backup is insanely slow. The first 500 MB took 15 minutes. Drive is not formatted as encrypted. (In a previous try the drive was formatted as encrypted. Backup timed out after 4GB and 24 hours.

What do I need to check or do to make this work like a disk drive, rather than a carrier pigeon?

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I’m happy to say the system started working a day or two later. It could have been reseating or replacing the cable, or something to do with Time Machine, or just something random.

It has gotten flaky at least 3 times since, becoming inaccessible to the iMac. In each case dismounting and/or remounting was required for the data to become visible. Just not very encouraging.

I did see there’s a new ‘security download’ available as of a bit earlier in the year. I downloaded it once, but haven’t installed it yet (that I can recall; not sure how you can tell).

Anyway, it’s been running more or less reliably for a month, so I guess that’s better than taking it back.