My friend has an iMac 2015 where he boots from an external USB 3.0 SSD. It worked well under HFS+ under High Sierra but boots slower under Mojave. The SSD has been bought in november 2018. The last days, the iMac hanged at startup, except with safe boot. We tried SMC reset, safe boot and disk utility but it didn't solve the problem. DU says the disk is OK. However, when using de single user mode (command + S) we ran fsck -fy and it booted afterwards. Now the problem is gone. We booted 10 times under the normal way today without a problem. However, we are still not sure if everything is OK. Is there anyone who has a suggestion what to do if the lockup happens again?