Is it possible or likely for malware or a virus to be transferred by plugging a USB hard drive into my Mac?
Possible: Yes.
Everything is possible. You may have an infection as yet undiscovered which can make the leap, or one buried in some seemingly innocuous file you're about to copy over.

Likely: No.
With no autorun, it is highly unlikely anything can make the leap of its own accord.

Mitigation...
Get an antivirus, there are plenty about; well-known from the Windows world.
Malwarebytes is still free, &amp; there are free versions of some of the others.  

AV-TEST.org is one of the closest things to an actual authority on the subject. They ran some tests on Mac antivirus apps recently &amp; their findings are in that link. Be aware that some of the slower performers really do give a noticeable speed impact even on my 12-core Mac Pro.
Malwarebytes isn't on the list, but tends to be regarded as "something to have as well as an antivirus".

No protection is ever going to be absolutely 100% guaranteed perfect, but just because "everybody" says Macs can't catch viruses doesn't mean they're actually right, &amp; you're always better safe than sorry.