A flash storage device is divided into blocks (the size of a block depends on the device, usually between 64 and 256KB). The number of times a block can be erased is limited : from 10'000 times to hundreds of thousands of times, depending on the device. Usually, you get what you pay for. Cheap quality deals don't really exist.
Reading and writing doesn't affect the life of a block, it's only the erasing that wears out, not the writing. The reason people think that writes are limited is because one needs first to erase a block before writing to it.
Because blocks can only be erased a certain number of times, manufacturers reserve spare blocks that are not visible to the user. When a blocks is failing (can't be erased properly), the firmware of the device maps a new block from the spare ones. This is why performance degrades over time and when there are no more spare blocks available, users starts to see corrupted blocks (leading to corrupted files).
High grade flash memories are much higher rated in terms of erase capacity. The worse flash memories are found in SD cards. USB sticks tend to be better and SSD drives are at the top of the food chain in terms of reliability (i.e. number of erase cycles). In fact, a good SSD drive today is more reliable in the long run than a classic magnetic hard disk.
Please not that what I say above applies to any flash device (including built-in flash found in phones or anywhere else).
If you want to maximize reliability, check the specs of a flash device in terms of write cycles and buy accordingly, or use a camera featuring dual SD card slots and configure the camera to write the same file on both cards.
Reading and writing doesn't affect the life of a block, it's only the erasing that wears out, not the writing. The reason people think that writes are limited is because one needs first to erase a block before writing to it.
Because blocks can only be erased a certain number of times, manufacturers reserve spare blocks that are not visible to the user. When a blocks is failing (can't be erased properly), the firmware of the device maps a new block from the spare ones. This is why performance degrades over time and when there are no more spare blocks available, users starts to see corrupted blocks (leading to corrupted files).
High grade flash memories are much higher rated in terms of erase capacity. The worse flash memories are found in SD cards. USB sticks tend to be better and SSD drives are at the top of the food chain in terms of reliability (i.e. number of erase cycles). In fact, a good SSD drive today is more reliable in the long run than a classic magnetic hard disk.
Please not that what I say above applies to any flash device (including built-in flash found in phones or anywhere else).
If you want to maximize reliability, check the specs of a flash device in terms of write cycles and buy accordingly, or use a camera featuring dual SD card slots and configure the camera to write the same file on both cards.