Yes if you set bios>sata operation to AHCI and save change.  Dell default is raid. SATA OperationAllows you to configure the internal SATA hard-drive controller. The options are:  
i have a latitude 5591, it's configured with the original M.2 SSD in the bay to the left of the battery.this bay could be used with a 2.5" SATA drive with the optional 6NVFT SATA adapter - which i do have.I'd like to install a spare 2.5" SATA drive (SSD, not spindle) in addition to the M.2 that's already in there, but the M.2 takes up about 1/3'rd of the space of the 2.5" drive.  I have opened up the 2.5" SSD and found that the actual electronics would allow me to shorten the 2.5" drive case so i can get both it and the M.2 to fit the space. so the $64 question is - will BIOS recognize and allow access to both drives?is there anything special i need to configure in the BIOS to allow it? Thanks
two drives in a latitude 5591