In a dual-drive system, the system manufacturer will install a small SSD primary drive (C:) for the operating system and apps, and add a larger spinning hard drive (D: or E:) for storing files. This works well in theory; in practice, manufacturers can go too small on the SSD. Windows itself takes up a lot of space on the primary drive, and some apps can't be installed on other drives. Some capacities may also be too small. For example, you can install Windows on a SSD as small as 16GB, but there will be little room for anything else. In our opinion, 120GB to 128GB is a practical minimum size for the C: drive, with 256GB or more being even better. Space concerns are the same as with any multiple-drive system: You need physical space inside the PC chassis to hold two (or more) drives.