For short-term backups flash memories are very reliable. They are unreliable for long-term storage without using them. They are guaranteed for only one year of

For short-term backups flash memories are very reliable. They are unreliable for long-term storage without using them. They are guaranteed for only one year of cold storage, because their memory-cell capacitors will slowly discharge. When the flash memories are new, i.e. they have not been written multiple times, they may hold their data for more than the guaranteed one year, but for archival purposes you should never count on being lucky. You can extend the storage time on flash memories by reading them periodically, because if they have good firmware for their controllers then they will automatically rewrite any sector that has been read with errors.