Free storage = total storage - used storage. Add the sizes of all new files that must fit together on one drive. Compare that sum with free storage, not total storage. An exact fit is enough in this paper model. A larger drive can have less room left. Capacity alone does not measure speed.
Worked example
A sample drive has 30 MB total and 18 MB used, leaving 12 MB free. Two new files need 5 MB and 7 MB: 5 + 7 = 12 MB. Both fit exactly on that drive; 0 MB remains. The example drive is not either practice drive.
Two Drives, One Delivery
Drive B has 48 MB total storage and 29 MB used. Drive A has 100 MB total storage and 83 MB used. A new drawing needs 8 MB and a new sound file needs 11 MB. Keep both new files together on one drive. All quantities use the same MB unit. Sizes already include all space needed; nothing else is added or removed. No splitting, compression or extra overhead is modeled. Use these invented reports only; do not delete real files.