Final answer:
The coworker likely forgot to initialize, partition, and format the new hard drive through Disk Management, which must be done before the drive will appear in File Explorer.
Step-by-step explanation:
If a coworker has installed a second hard drive in their Windows computer and does not see the drive in File Explorer, they may have forgotten a key step after physically installing the drive. The drive likely needs to be initialized, partitioned, and formatted before it will appear in File Explorer.
To do this, one must open Disk Management by right-clicking on 'This PC' or 'Computer' on the desktop, select 'Manage', and then choose 'Disk Management'. The new drive should appear as 'Unallocated Space'.
They must then right-click on the unallocated space, create a new simple volume, and follow the wizard to initialize, partition, and format the drive with a file system such as NTFS or exFAT. Once this process is complete, the drive will be assigned a drive letter and will appear in File Explorer.