The Dell Precision 3561 Mobile Workstation Supports Either 2 or 3 Drives Depending on the Battery Installed. (One 2.5" Solid State Drive or Hard Drive with the Smaller 4-cell 64Whr Battery Installed and/or Two M.2 NGFF (2280) or (2230) Solid State Drives) or (Two M.2 NGFF (2280) or (2230) Solid State Drives with the Larger 6-cell 97Whr Battery Installed) Note: Models that have the larger 6-cell 97Whr battery installed do not support 2.5" Drives due to the larger battery occupying the 2.5" Drive bay. Both of the M.2 slots support M.2 (2280) or (2230) Drives by moving the SSD holder screw to accommodate the form factor. If the 2.5" Drive was not installed from Dell in machines with the smaller 64Whr battery installed, you will need the 2.5" Drive caddy p/n 096GVC and cable p/n 0PCX32 for installation. (Both M.2 slots support Generation 4.0 x4 and 3.0 x4 PCIe NVMe Solid State Drives)