My Book Studio USB 3.0 no longer recognised by iMac (but mounts fine on other Macs)

My Book Studio 3TB 3.0 USB used to work fine as TimeMachine on iMac (Retina 5K, 27-inch, Late 2014) running Yosemite. It is no longer recognised after upgrading to El Capitan. It still mounts just fine on MacBook running Yosemite. Used DiskUtility on that machine to erase and reformat the disk. The iMac still does not recognise it. However, Transcend SSD mounts fine on iMac.
Any hints of what can be wrong?
Thanks in advance.

I’d recommend replacing the Micro USB 3.0 cable with a Micro 2.0 cable in order to force USB 2.0 mode. If it works, then USB 3.0 drivers associated with the unit’s 3.0 mode may have been corrupted during the update.

This did it! (Despite my initial disbelief).

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Glad to to know.