For an external usb hard drive attached to a computer, which is more suitable: a writethrough cache or a block cache

Disk access is way slower than memory access. Caching is a technique to improve the disk access time.  Block cache is a caching technique which reduces disk accesses time.  Here a portion of disk is bought to cache for reading and a modified blocks are first changed in cache but reflected in the disk at one go.  On the other hand with write through caching each modified block is written to cache and at the same time it is written to the disk.  Write- through caching requires more disk I/O so they can have a negative effect on the performance. 

block cache

Explanation:

    An integrated cache is a layer of memory that automatically caches frequently accessed data through the source database.

    Block cache is used to store blocks read from disks. This means that a small buffer cache will cause Oracle to cache data blocks following the LAST RECENTLY USED list, and depending on how often this happens, can lead to a drop in performance.

    There is no magic to sizing the block cache, what you usually do is estimate an initial size and monitor the hit, if it is not optimal, you will need to lengthen and repeat the monitoring cycle.