SSDs are actually dum as compared to HDDs. During write/delete operations they accumulate all sorts of garbage, becoming slower and slower. HDDs keep track of every single sector on their platters and keep a map of them, since they have access to the file system structures, including the list of unused clusters. SSDs don't have access to the file system structures. The quote below is not from me but from Wikipedia
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http://en.wikipedia.org/wiki/TRIM):
"In computing, a TRIM command allows an operating system to inform a solid-state drive (SSD) which blocks of data are no longer considered in use and can be wiped internally. While TRIM is frequently spelled in capital letters, it is a command name, not an acronym.
TRIM was introduced soon after SSDs started to become an affordable alternative to traditional hard disks. Because low-level operation of SSDs differs significantly from hard drives, the typical way in which operating systems handle operations like deletes and formats (not explicitly communicating the involved sectors/pages to the underlying storage medium) resulted in unanticipated progressive performance degradation of write operations on SSDs. TRIM enables the SSD to handle garbage collection overhead, which would otherwise significantly slow down future write operations to the involved blocks, in advance."
