Any back up source can become corrupted
so always back up to two different sources.
Memory sticks have a finite number of read write cycles The memory in flash drives is commonly engineered with Multi-level cell (MLC) based memory that is good for around 10,000 writes; but some flash drives have Single-level cell (SLC) based memory, good for around 100,000 writes. There is virtually no limit to the number of reads from such flash memory.
A well worn USB may be write protected to help ensure the life of individual cells; however, the USB connector hardware can withstand only around 1,500 cycles. from here