7060 SFF-Adding 2nd HDD

This is the specifications for OptiPlex 7060 SFF  https://www.dell.com/support/manuals/en-us/optiplex-7060-sff/opti_7060_sff_setup_specs_manual/storage-combinations?guid=guid-5624af8d-25c9-486e-8445-1e8426632fc1&lang=en-us

If your current boot drive is an M.2, you can add a 3.5" HDD as second drive.  Otherwise, you can follow this storage combination chart to add the second drive accordingly.

You can install either a 3.5" HDD or two 2.5" HDD/SSD following the instructions from the service manual  https://www.dell.com/support/manuals/en-us/optiplex-7060-sff/opti_7060_sff_service_manual/removing-the-hard-drive-and-optical-drive-module?guid=guid-0392d94f-8a5f-41de-b4bf-9c54c4e7d384&lang=en-us

To install a 3.5" HDD, you will need drive caddy with part number H8V8K

To install one or two of 2.5" HDD/SSD, you will need drive caddy with part number F3TJ0

When using dual 2.5" drives, you will also need a SATA power splitter cable.  Any part number N701D or 247PN should work.

besides Chino's great detailed answers, if you already have a 3.5 hdd and it is thin (20 mm), you might be able to add a second thin 3.5 hdd just by piggybacking the two thin hdd.

You'll also need a SATA data cable.