Memory access time is how long it takes for a character in RAM to be transferred to or from the CPU. Fast RAM chips have an access time of 10 nanoseconds (ns) or less. See SDRAM
Disk access time is how long it takes to obtain the first data character after initiating a request. It includes the time to move the read/write head to the track (seek time) and time to rotate the platter to the sector (latency). Disk access time is always given as an average, because seek time and latency vary depending on the current position of the head and platter.
Disk vs. SSD
While access times of fast hard disks are typically from 5 to 10 milliseconds, solid state drive (SSD) access times are in the 25 to 100 microsecond range. SSDs are as much as 100 times faster because there is no mechanical seek time or latency associated with flash memory storage. However, access times for all storage types are only one metric. Channel speed (transfer rate) and caching contribute to overall storage performance. See cache
, seek time