My Mac takes a long time to boot.

shut down and disconnect all external devices except OEM mouse and keyboard

reset the smc

How to reset the System Management Controller (SMC) on your Mac - Apple Support

repeat 3 times

reset the nvram

How to reset NVRAM on your Mac - Apple Support

repeat 3 times

boot into recovery mode

https://support.apple.com/en-us/HT201314

bring up disk utility

select your boot drive and click "First Aid"

if repairs are needed the utility should make them. if there are problems that can not be fixed there is a more serious problem.

boot into safe mode

Use safe mode to isolate issues with your Mac - Apple Support

then reboot normally with everything except OEM mouse and keyboard disconnected

For futher analysis you may post an Etrecheck report. The link is http://www.etrecheck.com

Before trashing the disk, try an alternate cable. Sometimes the cable fails. It should be an easy test, assuming you have a similar cable you can experiment with.

Just a note, 3 TB Disks are the shortest lived of all sizes.

:)