External drives still take minutes to mount or sometimes never mount automatically. Sometimes I can manually mount them from Disk Utility. Other times I need to

External drives still take minutes to mount or sometimes never mount automatically. Sometimes I can manually mount them from Disk Utility. Other times I need to reboot or disconnect other drives first. I've encountered failing USB disks which appear to work fine on macOS, but suddenly operations take minutes to perform and can block operations system wide. They don't report errors, and they don't show as CPU use in any program, but everything on the system slows to a crawl when you hit sensitive spots on the disk. It's worth removing external storage devices and adding them back one by one any time you have a hanging or slow Mac to diagnose. Apple hates to scare or confuse users, but if a disk drive is taking a four orders of magnitude more time to do operations than it used to, I'd like my computer to please let me know.