[GUIDE] Upgrade a working My Cloud to a bigger HDD (easy steps)

“mkswap: cannot open /dev/sdg1: Device or resource busy” kept preventing me from a successful implementation of this instruction for a 2nd Gen. (WDBCTLxxxxxxx-10) My Cloud.
If ignoring this message, it got stuck in Safe mode with a firmware version 1.0.1.007 (as described in WD MyCloud Stuck in safe Mode) and got stuck in updating a firmware forever (even after performing the 40-second factory reset ).
Any hints would be appreciated.