I've had to use similar workarounds in the past to unmount disks, without ever really knowing why. I opened Activity Monitor and was just confused - it kept jumping around so much I couldn't focus on anything.  I wonder if something that always runs in the background was scanning your flash drive? Spotlight? An anti-virus app?
iMac (19,1, 3.1 GHz i5, 12.7.4, 40 Gb RAM); MacBook Air (1.8 Ghz, 8 Gb RAM, 10.14.6, 256 Gb SSD) Vodafone router and Devolo Wi-Fi Extender, Canon TS8351 printer/scanner.
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